Chemical Engineering Principles and Applications 9783031278785, 9783031278792

Citation preview

Nuggenhalli S. Nandagopal

Chemical Engineering Principles and Applications

Chemical Engineering Principles and Applications

Nuggenhalli S. Nandagopal

Chemical Engineering Principles and Applications

Nuggenhalli S. Nandagopal Bengaluru, Karnataka, India

ISBN 978-3-031-27878-5 ISBN 978-3-031-27879-2 https://doi.org/10.1007/978-3-031-27879-2

(eBook)

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Chemical Engineering Principles and Applications is the product of many years of teaching experience of review courses for the chemical engineering Professional Engineer (PE) license exam. The Principles and Practice of Engineering (PE) Exam in chemical engineering covers the entire spectrum of subjects in chemical engineering including momentum, heat, and mass transfer, mass and energy balances, chemical thermodynamics, and chemical reaction engineering. The teaching of any PE review course requires developing the art of presenting the subject matter in a clear and concise manner to practicing engineers, who may have been away from academic settings for several areas. The instructor has to build the concepts from ground zero, present them concisely, and enhance the understanding using relevant example problems that are field oriented and practical in nature. The most widely used comprehensive reference in chemical engineering is Perry’s Chemical Engineering Handbook. However, Perry’s handbook consists of 30 sections/chapters and 2559 pages! On the other end of the spectrum, books with lesser number of pages (200–300) typically address a single subject matter in the field of chemical engineering such as mass transfer, chemical reaction engineering, and so on. Chemical Engineering Principles and Applications, with 7 chapters and approximately 600 pages bridges this gap with emphasis on fundamental concepts and applications. It can be a “go to” reference for chemical engineering practitioners and students, as well as for those preparing for the PE-Chemical exam. Other salient features of this book include: • Comprehensive coverage of all aspects of mass and energy balances (the bread and butter of chemical engineering!) with a wide variety of illustrative examples and practice problems • In-depth coverage of relevant thermodynamic topics such as vapor-liquid equilibrium (VLE) and chemical equilibrium • Chemical engineering orientation in fluid mechanics with extended coverage on topics such as flow through packed beds and fluidized beds, typically not included in traditional books on fluid mechanics v

vi

Preface

• Focus on heat exchanger design, operation, and applications • Comprehensive coverage of all unit operations in mass transfer – gas absorption, stripping, distillation, humidification, drying, liquid-liquid extraction, leaching, adsorption, crystallization, and evaporation • Practical applications of chemical reaction engineering and kinetics • Introduction to Transport Phenomenon Comments and feedback are most welcome. Happy reading in your journey through chemical engineering! Bengaluru, Karnataka, India

Nuggenhalli S. Nandagopal

Contents

1

Mass and Energy Balances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Mass Balances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Fundamental Mass Balance Equation . . . . . . . . . . . . . . . 1.2.2 Mass Balances for Steady-State, Non-reactive Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Mass Balances for Steady-State, Reactive Systems . . . . . 1.2.4 Mass Balances for Systems with Recycle . . . . . . . . . . . . 1.2.5 Mass Balances for Systems with Bypass . . . . . . . . . . . . 1.2.6 Mass Balances for Non-reactive Unsteady-State Systems . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Energy Balances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Energy Balance for Closed Systems – First Law of Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Fundamental Energy Balance Equation . . . . . . . . . . . . . 1.3.3 Energy Balances for Reactive Processes . . . . . . . . . . . . 1.4 Mass and Energy Balances for Combustion Reactions . . . . . . . . . 1.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.2 Heating Value of Fuels . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.3 Stoichiometry of Combustion Reactions, Theoretical and Excess Air . . . . . . . . . . . . . . . . . . . . . . 1.4.4 Analysis of Combustion Products – Flue Gas Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.5 Dew Point of Combustion Products . . . . . . . . . . . . . . . . 1.4.6 Combustion of Coal – Use of Gravimetric Analysis of Coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 1 1 2 4 14 21 23 26 27 30 37 45 45 45 47 49 51 51 55 55 55 55 vii

viii

2

Contents

Practice Problem 1.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solutions to Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 1.10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

56 56 57 57 57 58 58 58 58 60 61 63 70 72 74 75 78 79 80

Chemical Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Equations of State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Ideal Gas Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Real Gas Equations of State . . . . . . . . . . . . . . . . . . . . . 2.2.3 Generalized Compressibility Chart – Compressibility Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Ideal Gas Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Thermodynamic Property Tables . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Steam Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Phase Diagram for Water . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Properties of Liquid–Vapor Mixtures . . . . . . . . . . . . . . . 2.3.4 Properties of Compressed Liquid . . . . . . . . . . . . . . . . . . 2.4 Thermodynamic Charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Mollier Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Pressure–Enthalpy (P–h) Phase Diagram . . . . . . . . . . . . 2.5 Vapor–Liquid Equilibrium (VLE) . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 VLE Relationships for Ideal Solutions . . . . . . . . . . . . . . 2.5.2 VLE Relationships for Non-ideal Systems . . . . . . . . . . . 2.5.3 Distribution Coefficients for VLE Involving Multicomponent Systems . . . . . . . . . . . . . . . . . . . . . . . 2.5.4 Generating VLE Curve Using Relative Volatility Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.5 Bubble Point and Dew Point Calculations . . . . . . . . . . . 2.5.6 Azeotropic Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . .

81 81 81 81 84 85 89 92 93 94 96 97 103 103 107 112 112 116 117 120 122 129

Contents

ix

2.6

133 134 136 137 137 139

Gibbs Free Energy Change and Phase Transitions . . . . . . . . . . . . 2.6.1 Clausius–Clapeyron Equation for Phase Transition . . . . . 2.7 Chemical Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.1 Characteristics of Chemical Equilibrium . . . . . . . . . . . . 2.7.2 Position of Equilibrium – Equilibrium Constant . . . . . . . 2.7.3 Equilibrium Calculations . . . . . . . . . . . . . . . . . . . . . . . 2.7.4 Disruptions to Equilibrium State – Le Chatelier’s Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.5 Gibbs Free Energy Change and Chemical Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.6 Temperature Dependence of Equilibrium Constant – The Van’t-Hoff Equation . . . . . . . . . . . . . . . Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.13 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.17 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solutions to Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 2.12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

144 145 148 149 149 149 149 149 149 150 150 150 150 150 151 151 151 153 154 154 154 154 155 155 155 159 160 161 162 163 164 164 165 165 166 167

x

Contents

Practice Problem 2.13 . . . . . . . . . . . . . . . . . . . . Practice Problem 2.14 . . . . . . . . . . . . . . . . . . . . Practice Problem 2.15 . . . . . . . . . . . . . . . . . . . . Practice Problem 2.16 . . . . . . . . . . . . . . . . . . . . Practice Problem 2.17 . . . . . . . . . . . . . . . . . . . . Practice Problem 2.18 . . . . . . . . . . . . . . . . . . . . Practice Problem 2.19 . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

167 169 170 171 172 172 173 174

Fluid Mechanics and Momentum Transfer . . . . . . . . . . . . . . . . . . . . 3.1 Role of Fluid Mechanics in Chemical Engineering . . . . . . . . . . . 3.2 Fluid Properties and Fundamentals . . . . . . . . . . . . . . . . . . . . . . . 3.3 Fluid Statics and Manometers . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Static Pressure and Pressure Head . . . . . . . . . . . . . . . . . 3.3.2 Differential Manometers . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Fluid Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Conservation of Mass – Continuity Equation . . . . . . . . . 3.4.2 Laminar Flow and Turbulent Flow Through Pipes . . . . . 3.4.3 Reynolds Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.4 Friction Head Loss and Pressure Drop for Fluid Flow in Pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Calculating the Friction Head Loss for Pipe Flow . . . . . . . . . . . . 3.5.1 Fanning Friction Factor . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Equations for Calculating Friction Factors . . . . . . . . . . . 3.5.3 Hagen–Poiseuille Equation . . . . . . . . . . . . . . . . . . . . . . 3.6 Flow Through Non-circular Cross Sections . . . . . . . . . . . . . . . . . 3.7 Flow Through Pipe Networks . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Head Loss Through Fittings and Valves – Minor Losses . . . . . . . 3.8.1 Velocity Head Method . . . . . . . . . . . . . . . . . . . . . . . . . 3.8.2 Equivalent Length Method . . . . . . . . . . . . . . . . . . . . . . 3.8.3 Other, Miscellaneous Minor Losses – Pipe Entrance, Pipe Exit, Change in Pipe Cross Section . . . . . 3.9 The Mechanical Energy Equation . . . . . . . . . . . . . . . . . . . . . . . 3.9.1 Bernoulli’s Equation . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9.2 Pump Power Equation . . . . . . . . . . . . . . . . . . . . . . . . . 3.9.3 Pump Performance Parameters . . . . . . . . . . . . . . . . . . . 3.10 Flow Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10.2 Pitot Tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10.3 Orifice Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10.4 Venturi Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10.5 Orifice Meter and Venturi Meter Comparison . . . . . . . . . 3.11 Flow Through Packed Beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11.1 Calculation of Pressure Drop in Packed Columns . . . . . . 3.11.2 Operation of Packed Columns – Generalized Pressure Drop Correlations and Flooding . . . . . . . . . . . .

175 175 175 180 180 182 185 185 187 188 190 190 191 195 198 199 202 203 204 205 206 209 214 215 219 221 221 221 223 225 228 229 229 232

Contents

xi

3.12 3.13

237 240 240 240

Fluidized Beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compressible Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13.2 Continuity Equation for Compressible Flow . . . . . . . . . . 3.13.3 Mach Number and Its Significance in Compressible Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13.4 Isentropic Gas Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13.5 Application of the Steady Flow Energy Equation for Isentropic Flows . . . . . . . . . . . . . . . . . . . . 3.13.6 Stagnation – Static Relationships . . . . . . . . . . . . . . . . . . 3.13.7 Isentropic Flow with Area Changes . . . . . . . . . . . . . . . . 3.13.8 Adiabatic Compressible Flow with Friction Loss . . . . . . Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solutions to Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 3.12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 General Equation for Modeling of Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Heat Transfer Modes . . . . . . . . . . . . . . . . . . . . . . . . . .

241 242 242 243 244 248 251 251 251 252 252 252 253 253 253 253 254 254 254 254 254 256 257 258 259 260 260 261 262 263 264 265 267 269 269 269 270

xii

Contents

4.2

Conduction Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Conduction Through a Rectangular Slab . . . . . . . . . . . . 4.2.2 Conduction Through a Cylindrical Wall . . . . . . . . . . . . 4.2.3 Conduction Through a Spherical Wall . . . . . . . . . . . . . . 4.3 Convection Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Convection Heat Transfer Resistance . . . . . . . . . . . . . . . 4.3.2 Dimensionless Parameters Used in Heat Transfer . . . . . . 4.3.3 Correlations Used in Calculating Convection Heat Transfer Coefficients . . . . . . . . . . . . . . . . . . . . . . 4.3.4 Typical Range of Convection Heat Transfer Coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.5 Overall Heat Transfer Coefficients in Conduction–Convection Systems . . . . . . . . . . . . . . . . . 4.3.6 Order of Magnitude Analysis to Determine the Value of Overall Heat Transfer Coefficients . . . . . . . 4.4 Heat Exchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Heat Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Log Mean Temperature Difference (LMTD) . . . . . . . . . 4.4.3 Overall Heat Transfer Coefficient for Heat Exchangers . . 4.4.4 Heat Exchanger Design Equation . . . . . . . . . . . . . . . . . 4.4.5 LMTD Correction Factors . . . . . . . . . . . . . . . . . . . . . . 4.4.6 Heat Exchanger Effectiveness . . . . . . . . . . . . . . . . . . . . 4.4.7 Effectiveness–NTU Method . . . . . . . . . . . . . . . . . . . . . 4.5 Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Stefan–Boltzmann’s Law of Thermal Radiation . . . . . . . 4.5.2 Non-Ideal Radiators – Gray Bodies . . . . . . . . . . . . . . . . 4.5.3 Absorptivity, Transmissivity, and Reflectivity . . . . . . . . 4.5.4 Radiation View Factor . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.5 View Factor Relationships . . . . . . . . . . . . . . . . . . . . . . 4.5.6 Calculation of Net Radiation Heat Transfer . . . . . . . . . . 4.5.7 Radiation Heat Transfer from a Small Object to an Enclosure, Both Being Gray Bodies . . . . . . . . . . . 4.5.8 Correction for Thermocouple Readings . . . . . . . . . . . . . Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 4.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 4.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 4.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 4.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 4.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solutions to Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 4.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 4.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 4.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 4.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 4.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

271 271 275 278 280 281 282 283 285 286 288 290 291 291 292 294 299 300 301 305 306 307 307 308 308 308 309 311 312 312 312 313 313 313 314 314 315 316 317 319 320

Contents

5

Mass Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Molecular Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Mass Transfer Coefficients and Interphase Mass Transfer . . . . . . 5.3.1 Interphase Mass Transfer . . . . . . . . . . . . . . . . . . . . . . . 5.4 Gas Absorption and Stripping . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Process Design of Absorption Columns . . . . . . . . . . . . . 5.5 Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.1 Batch Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.2 Continuous Distillation . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Psychrometrics and Psychrometric Chart . . . . . . . . . . . . . . . . . . 5.6.1 Moist Air Properties and Definitions . . . . . . . . . . . . . . . 5.6.2 Finding Moist Air Properties from Psychrometric Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 Cooling Towers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.1 Drying Mechanism and Rate of Drying . . . . . . . . . . . . . 5.8.2 Adiabatic Drying and Adiabatic Saturation Process . . . . 5.9 Liquid–Liquid Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.1 Ternary Phase Diagrams . . . . . . . . . . . . . . . . . . . . . . . . 5.9.2 Lever Rule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.3 Multistage Extractions . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 Leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10.1 Batch Leaching Process . . . . . . . . . . . . . . . . . . . . . . . . 5.10.2 Single-Stage Continuous Counterflow Leaching Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10.3 Multistage Continuous Counterflow Leaching . . . . . . . . 5.10.4 Determining Number of Stages Required Using the R-Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.11 Adsorption Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.11.1 Adsorption Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . 5.11.2 Continuous Multistage Counterflow Adsorption . . . . . . . 5.12 Crystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.12.1 Crystallization Process Calculations . . . . . . . . . . . . . . . 5.13 Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.13.1 Single-Effect Evaporator . . . . . . . . . . . . . . . . . . . . . . . . 5.13.2 Multiple-Effect Evaporator . . . . . . . . . . . . . . . . . . . . . . Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xiii

323 323 324 325 326 331 332 345 345 347 361 361 363 365 369 369 374 376 378 379 381 389 390 392 394 395 399 399 401 405 406 410 410 413 420 420 421 421 422 422 423 423

xiv

6

Contents

Practice Problem 5.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solutions to Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 5.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

423 424 424 424 427 429 431 432 433 435 436 438 440

Chemical Reaction Engineering and Kinetics . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Key Definitions in Chemical Reaction Engineering . . . . . . . . . . . 6.3 Arrhenius Equation – Effect of Temperature on the Rate Constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Constant Volume and Variable Volume Systems . . . . . . . . . . . . . 6.5 List of Useful Integrals in Reactor Design . . . . . . . . . . . . . . . . . 6.6 Reactor Types, Design Equations, and Applications . . . . . . . . . . 6.6.1 Batch Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.2 Mixed Flow Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.3 Plug Flow Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.4 Comparison of Volumes of Mixed Flow Reactor and Plug Flow Reactor . . . . . . . . . . . . . . . . . . . 6.6.5 CSTRs in Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solutions to Practice Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practice Problem 6.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

441 441 441 445 448 450 450 451 454 459 462 464 467 467 467 467 467 468 468 468 468 469 470 471 472 473 474

Contents

7

Transport Phenomenon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Mass Transfer Correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Analogies Between Momentum, Heat, and Mass Transfer . . . . . . 7.3.1 Stanton Number for Heat Transfer . . . . . . . . . . . . . . . . . 7.3.2 Colburn Factor for Heat Transfer . . . . . . . . . . . . . . . . . 7.3.3 Stanton Number for Mass Transfer . . . . . . . . . . . . . . . . 7.3.4 Colburn Factor for Mass Transfer . . . . . . . . . . . . . . . . . 7.3.5 Chilton–Colburn Analogy Between Momentum, Heat, and Mass Transfer . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xv

475 475 476 483 483 483 484 484 485 493

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495

Chapter 1

Mass and Energy Balances

1.1

Introduction

Mass and energy balances form the foundations of chemical engineering [6]. Mass balances are based on the principle of conservation of mass while energy balances are based on the principle of conservation of energy, also the first law of thermodynamics. In the analysis of any problem or situation in chemical engineering, the initial step is always to perform mass and energy balances to generate the required equations to solve the problem. The following section looks at different aspects of mass balances beginning with the fundamental mass balance equation, which can be simplified for specific situations.

1.2

Mass Balances

Mass balances are based on the principle that the mass must be conserved. The mass balance principle can be applied to each species as well as to the overall system. Mass balances are derived from the fundamental mass balance equation presented here [5].

1.2.1

Fundamental Mass Balance Equation

The fundamental mass balance equation applied to an open system, an entity with well-defined boundaries, is as follows.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 N. S. Nandagopal, Chemical Engineering Principles and Applications, https://doi.org/10.1007/978-3-031-27879-2_1

1

2

1

Mass and Energy Balances

Input þ Generation - Output - Consumption = Accumulation

ð1:1Þ

Each term in Eq. 1.1 is explained here. Input refers to the mass flow rate of species entering through the system boundaries. Generation refers to the rate of production of species within the system boundaries. A species is usually generated due to chemical reaction(s) in the system. Output refers to the mass flow rate of species leaving through the system boundaries. Consumption refers to the rate of decrease of mass of species within the system boundaries. Generally, a species is consumed due to chemical reaction(s) in the system. Accumulation is the rate at which the mass of a species increases or decreases within system boundaries. In steady-state situations, there is no accumulation of species within the system. The fundamental mass balance equation (Eq. 1.1) can be applied to specific species, and it can also be applied to the input stream as a whole. Further, Eq. 1.1 can be simplified for specific situations as follows.

1.2.2

Mass Balances for Steady-State, Non-reactive Systems

In steady-state systems, there is no accumulation of species within the system. Further, the generation and consumption terms are zero in a system with no reactions. Therefore, Eq. 1.1 simplifies to the following basic equation. Input = Output

ð1:2Þ

Many mass balance problems involving only physical changes can be solved using Eq. 1.2. Since the equation to solve such problems is so simple, most of the effort is spent in using the right strategy and approach to solve the problem. The general strategy for solving mass balance problems is outlined here [5]. 1. Draw a well-defined schematic for the problem including all the known information. 2. Assign algebraic symbols to unknown quantities. 3. Generate algebraic equations using mass balances for species and overall mass balances. 4. Solve the algebraic equations for the unknowns.

1.2

Mass Balances

3

Example 1.1 A chemical process requires a 40% hydrochloric acid solution at a rate of 1000 kg/hr. A concentrated solution of 70% hydrochloric acid is available. Determine the mass flow rates of the concentrated acid and diluting water required to generate the acid stream required for the chemical process. Solution Draw the schematic diagram for the process as shown.

This is a steady-state, non-reactive situation, where Eq. 1.2 can be applied. Mass balance for HCl based on Eq. 1.2: Input of HCl = Output of HCl

Að0:70Þ þ Wð0Þ = 1000 A = 1000

kg hr

kg ð0:4Þ ) hr

0:4 = 571:4 kg=hr 0:70

Overall mass balance based on Eq. 1.2: Overall input = Overall output )

A þ W = P = 1000 W = 1000

kg ) hr

kg kg kg - A = 1000 - 571:4 = 428:6 kg=hr hr hr hr

Example 1.2 The following data is available for a component from the bottom tray and from the kettle reboiler of a distillation column: vapor mole fraction from the bottom tray = 0.73, liquid mole fraction from the bottom tray = 0.88, vapor mole fraction from the reboiler = 0.77. If the ratio of the liquid flow from the bottom tray to the vapor flow from the reboiler is 3.50, determine A. the boil-up ratio for the column B. the mole fraction of the component in the bottom product. Solution Draw the schematic diagram of the bottom of the distillation column and the kettle reboiler as shown.

4

1

Mass and Energy Balances

Material balance around the red, dashed envelope results in the following equations. A. Overall mass balance: Mass flow in = mass flow out

) L = V þ B ) B = L - V, Since

L = 3:5 ) V

3:5V = V þ B ) V = 0:4 ðboil - up ratioÞ B B. Mass balance for the less volatile component around the red dashed envelope results in the following equations Mass flow of component in = mass flow of component out ) Lx = Vy þ BxB , Since B = L - V ) Lx = Vy þ ðL - VÞxB )

xB =

1.2.3

L Lx - Vy ðV Þx - y ð3:50Þð0:88Þ - 0:77 = = 0:924 = L 3:50 - 1 ðL - VÞ V -1

Mass Balances for Steady-State, Reactive Systems

For reactive systems, the mass balance equation must include the generation and consumption terms. Products are generated while reactants are consumed. Further, the accumulation term is zero for steady-state reactive systems. Therefore, the mass balance equation for steady-state reactive systems is

1.2

Mass Balances

5

Input þ Generation = Output þ Consumption

ð1:3Þ

Also, the stoichiometric reaction equation must be considered while solving problems on reactive systems [5, 6].

1.2.3.1

Limiting and Excess Reactants

The stoichiometric reaction equation forms the basis for determining the limiting and excess reactants in the feed stream [5]. Consider the reaction of methane with oxygen to form carbon dioxide and water vapor. The stoichiometric equation for the reaction is CH4 þ 2O2 → CO2 þ 2H2 O On the reactant side, the reactant that is completely consumed is the limiting reactant whereas the reactant that is in excess of the stoichiometric requirement is the excess reactant. Supposing the reactor feed consists of 10 moles of methane and 30 moles of oxygen, then all moles of methane will be consumed and hence methane is the limiting reactant. As per the stoichiometric equation, one mole of methane requires two moles of oxygen for completing the reaction. Hence the stoichiometric requirement of oxygen for 10 moles of methane is 20 moles of oxygen. However, since the feed consists of 30 moles of oxygen, there are 10 moles of oxygen in excess of the theoretical requirement and hence oxygen is the excess reactant. The percentage excess of the excess reactant in the feed can be determined using Eq. 1.4. Percent Eexcess of excess reactant =

N actual - N stiochiometric 100 N stiochiometric

ð1:4Þ

Equation 1.4 can be applied for determining the percent excess of oxygen in the feed as shown here. Percent Eexcess of oxygen =

30 - 20 100 = 50% 20

Example 1.3 illustrates the application of the mass balance equation (Eq. 1.3) for steady-state reactive systems. Example 1.3 Methane reacts with oxygen to produce carbon dioxide and water. The feed stream to the reactor consists of 20 kmol/min of methane and 50 kmol/min of oxygen. However, only 80% of the limiting reactant is consumed in the reactor, that is, the reactor achieves 80% conversion of the limiting reactant. Determine the molar flow rate of each component in the product stream.

6

1

Mass and Energy Balances

Solution Write the stoichiometric equation for the reaction. CH4 þ 2O2 → CO2 þ 2H2 O From the stoichiometric equation, 20 kmol/min of methane fed to the reactor will require 40 kmol/min of oxygen. Since 50 kmol/min of oxygen is fed to the reactor, oxygen is in excess of the theoretical requirement and methane is the limiting reactant. Since the conversion of the limiting reactant (methane) is 80%, the product stream will consist of unreacted methane, unreacted oxygen, and the reaction products, carbon dioxide and water vapor. Draw the schematic for the reactor as shown.

Calculate the moles of methane and oxygen reacting for 80% conversion of methane. Moles CH4 reacting = 0:8 × 20

kmol = 16 kmol= min min

Determine the moles of O2 reacting and the moles CO2, H2O produced from the stoichiometric equation. Moles O2 reacting = 2(moles CH4 reacting) = 2 × 16 kmol min = 32 kmol= min Moles CO2 produced = moles CH4 reacting = 16 kmol/min Moles H2O produced = 2(moles CH4 reacting) = 2 × 16 kmol min = 32 kmol= min The mass balance equation (Eq. 1.3) for steady-state reactive systems is Input þ Generation = Output þ Consumption Apply the preceding equation to each species of reactants and products to obtain the molar flow rate of each component in the product stream. CH4 : 20

kmol kmol þ 0 = N_ CH4 ,out þ 16 ) min min

N_ CH4 ,out = 4 kmol= min

O2 : 50

kmol kmol þ 0 = N_ O2 ,out þ 32 ) min min

N_ O2 ,out = 18 kmol= min

1.2

Mass Balances

7

CO2 : 0 þ 16

kmol = N_ CO2 ,out þ 0 ) min

N_ CO2 ,out = 16 kmol= min

H2 O : 0 þ 32

kmol = N_ H2 O,out þ 0 ) min

N_ H2 O,out = 32 kmol= min Note In this problem, total moles of products = total moles of reactants = 70 kmol/ min. This is because three moles of reactants produce three moles of products as per the stoichiometric equation. Hence, In reactive systems, mass is always conserved whereas moles are not always conserved. Moles are conserved only if the moles of reactants and products are equal.

1.2.3.2

Multiple Reactions – Conversion, Extent of Reaction, Selectivity, and Yield

In reactive systems, undesirable side reactions can take place along with the reaction that produces the desired product. In such situations, the parameters conversion, selectivity, and yield are defined [1, 5, 6] and calculated to obtain a measure of the effectiveness of the system in producing the desired product with minimal consumption of expensive reactant(s). Conversion The term conversion is usually applied to the limiting reactant, and it can be expressed in terms of fractional conversion or percentage conversion. If ṄF is the molar flow of the limiting reactant in the feed, and Ṅ is the molar flow of the limiting reactant in the outlet stream, then the formulas for fractional and percent conversion are: N_ N_ F - N_ = Reacted N_ F N_ F

ð1:5Þ

N_ F - N_ N_ Reacted 100 = 100 _N F N_ F

ð1:6Þ

Fractional Cconversion =

Percent Cconversion =

Extent of a Reaction The extent of a reaction is a measure of the completeness of a reaction. If ṄiF is the molar flow rate of species i in the feed and if Ṅi represents the molar flow rate of species i in the product, and if ±νi is the stoichiometric coefficient of species i, with plus sign used for product species and minus sign used for reactant species, then the extent of the reaction, ξ_ is defined as follows.

8

1

N_ - N_ iF ξ_ = i ± νi

Mass and Energy Balances

ð1:7aÞ

Note For inerts, the stoichiometric coefficient is, νi = 0 Equation 1.7a can also be written as shown in Eq. 1.7b. N_ i = N_ iF ± νi ξ_

ð1:7bÞ

Equation 1.7b can be written for multiple reactions as shown in Eq. 1.7c and each independent reaction has its own extent. νij ξ_ j

N_ i = N_ iF ±

ð1:7cÞ

j

Selectivity In case of multiple reactions producing ṄD moles of desired product per unit time and ṄU moles of undesired product per unit time, then the selectivity is defined as the ratio of the moles of desired product formed to the moles of undesired product formed. Selectivity =

N_ D N_ U

ð1:8Þ

Yield The yield of a product species is a measure of how much desired product is formed due to the reaction. If ṄD moles of desired product is formed per unit time and if ṄD,ideal represents the moles desired product formed per unit time under ideal conditions, that is, when there are no side reactions and there is complete conversion of limiting reactant, then the fractional yield is defined as the ratio of the moles of desired product formed per unit time to the moles of desired product formed per unit time under ideal conditions. Yield =

N_ D _N D,ideal

ð1:9aÞ

The fractional yield can be multiplied by 100 to obtain the percentage yield of the product. Percent yield = 100

N_ D

N_ D,ideal

ð1:9bÞ

Yield Based on the Moles Reacted The yield based on the moles reacted is the molar rate of formation of the desired product divided by the moles of the limited reactant reacting per unit time.

1.2

Mass Balances

9

Yield based on moles reacted =

N_ D _N R, lim reactant

ð1:10Þ

Note In Eqs.1.5, 1.6, 1.7a, 1.7b, 1.7c, 1.8, 1.9a, 1.9b and 1.10, the molar flow rates, Ṅ, used for continuous (or flow) reactors can be replaced by the number of moles, N, for batch reactors. Example 1.4 The raw materials used in the production of Acrylonitrile are, propylene (C3H6), ammonia (NH3), and oxygen (O2). Oxygen is sourced from air. The complete reaction equation is: 2C3 H6 þ 2NH3 þ 3O2 → 2C3 H3 N þ 6H2 O Molecular weights (lbm/lbmol): C3 H6 = 42, NH3 = 17, O2 = 32, C3 H3 N = 53, H2 O = 18, air = 29 It is desired to produce 600 lbm/hr of acrylonitrile using a reactor feed mixture with molar composition 10% propylene, 12% ammonia, and 78% air. By recycling unreacted propylene, an overall conversion of 60% propylene is achieved. Determine: A. the molar flow rate of the feed required. B. the percent excess of each excess reactant. C. the lbm/hr of water to be removed from the product mixture. Solution Basis: 600 lbm/hr of acrylonitrile product. Calculate the molar rate of production of acrylonitrile.

)

600 lbm m_ C3 H3 N hr = = 11:32 lbmol=hr = N_ P,C3 H3 N lbm M C3 H3 N 53 lbmol

Let N_ F be the total molar flow rate of feed. Draw the schematic diagram for the reactor as shown. Subscript F represents the feed and subscript P represents the product.

A. From the stoichiometric equation, 2 mols of propylene produces 2 mols of acrylonitrile. Therefore,

10

1

Mass and Energy Balances

Rate of consumption=reaction of propylene = rate of production of acrylonitrile = 11:32 lbmol=hr The overall conversion of propylene is 60%. Use the definition of percent conversion (Eq. 1.6) to obtain the molar flow rate of propylene required in the feed. Percent conversion =

N_ C3 H6 ,reacted N_ C3 H6 ,fed = %conversion 100

=

N_ C3 H6 ,reacted 100 ) N_ C3 H6 ,fed

11:32 lbmol hr 60% 100

= 18:87 lbmol=hr

From the given feed composition, propylene is 10 mole% of the total feed. Hence, 0:10N_ F = N_ C3 H6 ,fed = 18:87 lbmol=hr ) N_ F = 188:7 lbmol=hr feed B. From the feed composition, determine the molar flow rates of ammonia and oxygen in the feed. The mole fraction of oxygen in air is 0.21. lbmol = 22:64 lbmol=hr N_ NH3 ,fed = 0:12N_ F = ð0:12Þ 188:7 hr lbmol N_ O2 ,fed = ð0:21Þ 0:78N_ F = ð0:21Þð0:78Þ 188:7 hr = 30:91 lbmol=hr Based on the production rate of 11.32 lbmol/hr of acrylonitrile, from the stoichiometric equation, the molar flow rates of the reactants reacting are: N_ NH 3 ,reacted = N_ C3 H 6 ,reacted = 11:32 lbmol=hr

lbmol 3 lbmol O2 ð1:5Þ = 11:32 hr 2 lbmol C3 H6 = 16:98 lbmol=hr

N_ O2 ,reacted = N_ C3 H6 ,reacted

Based on the production of 600 lbm/hr of acrylonitrile, calculate the percent excess of each reactant using Eq. 1.4.

1.2

Mass Balances

11

Percent excess of excess reactant =

N actual - N stioch: N stiochiometric

100

N_ actual,C3 H6 - N_ stioch:,C3 H6 100 Percent excess of propylene = N_ stioch:,C3 H6 lbmol lbmol 18:87 - 11:32 hr hr = 100 = 66:7% lbmol 11:32 hr N_ actual,NH3 - N_ stioch:,NH3 Percent excess of ammonia = 100 N_ stioch:,NH3 lbmol lbmol - 11:32 hr hr 100 = 100% = lbmol 11:32 hr N_ actual,O2 - N_ stioch:,O2 100 Percent excess of oxygen = N_ stioch:,O2 lbmol lbmol 30:91 - 16:98 hr hr 100 = 82% = lbmol 16:98 hr 22:64

C. Calculate the lbmol/hr of water produced by using the stoichiometric equation and the rate of production of acrylonitrile and then multiply the result by the molecular weight of water to obtain the lbm/hr of water to be removed from the product. m_ H2 O = N_ P,H2 O ðM H2 O Þ = N_ P,C3 H3 N

6 lbmol H2 O ðM H2 O Þ 2 lbmol C3 H3 N

lbmol lbm ð3Þ 18 hr lbmol = 611:3 lbm=hr

= 11:32

Example 1.5 Ethylene (C2H4) is produced by the dehydrogenation of ethane (C2H6). However, the hydrogen (H2) produced by the reaction can react with ethane to produce undesired methane (CH4). The two parallel reactions are shown here. C2 H6 → C2 H4 þ H2 ðR1Þ C2 H6 þ H2 → 2CH4 ðR2Þ Pilot plant studies have been conducted on the use of a new catalyst that will maximize the production of ethylene. The studies reveal the following molar composition of the product gases: ethane = 13%, ethylene = 38%, hydrogen = 27%, methane = 22%. Determine:

12

A. B. C. D. E.

1

Mass and Energy Balances

the molar feed rate (mol/s) of ethane to the reactor. the percent conversion of ethane. the percent yield of ethylene. the selectivity of ethylene relative to methane. the extents of the reactions 1 and 2.

Solution The product stream consists of unreacted ethane, ethylene, hydrogen, and methane. Since the product composition is known, take the basis of calculations as 100 mol/s of product gases. Basis 100 mol/s of product gases ) 13 mol/s of ethane, 38 mol/s of ethylene, 27 mol/s of hydrogen, and 22 mol/s of methane in the product gas. This is illustrated in the schematic diagram.

A. As per the stoichiometric equation for reaction (R1), Mols ethane consumed = mols ethylene produced = 38 mol=s: As per the stoichiometric equation for reaction (R2), 1 ðmols methane producedÞ 2 1 = ð22 mol=sÞ = 11 mol=s 2

Mols ethane consumed =

Apply the steady-state mass balance equation for reactive systems (Eq. 1.3, shown here reference) for ethane as shown. Note that generation term is zero for reactants. Input þ Generation = Output þ Consumption ðEq:1:3Þ

mols mols mols mols C2 H6 : N_ F þ 0 = 13 þ 38 þ 11 s s s s mols ) N_ F,C2 H6 = 62 s

1.2

Mass Balances

13

B. The total moles of ethane reacting in both reactions (R1) and (R2) is

38

mol mol þ 11 = 49 mol=s: s s

Calculate the percent conversion of ethane using Eq. 1.6. Percent conversion of ethane =

49 mol N_ Reacted s 100 = 100 = 79% 62 mol N_ F s

C. Note that under ideal circumstances all the ethane in the feed would be converted to ethylene, the desired product resulting in a theoretical yield of 100%. Therefore, N_ D,ideal = 62 mol=s: Calculate the percent yield of ethylene by using Eq. 1.9b. Percent yield = 100

N_ D

N_ D,ideal

= 100

38 mol s = 61:3% 62 mol s

D. Ethylene is the desired product, therefore, N_ D = 38 mol=s: Methane is the undesired product and therefore, N_ U = N_ CH4 ,prod: = 22 mol=s: Calculate the selectivity of ethylene relative to methane using Eq. 1.8. Selectivity =

38 mol N_ D s = = 1:73 22 mol N_ U s

E. Use ethane as the reference species. Ethane participates in both reactions (R1) and (R2). Apply Eq. 1.7c to ethane.

14

1

Mass and Energy Balances

N_ C2 H6 = N_ C2 H6 ,F - ξ_ 1 - ξ_ 2 mol mol _ ) 13 = 62 - ξ1 - ξ_ 2 s s mol mol ) ξ_ 1 þ ξ_ 2 = 62 - 13 s s = 49 mol=s Use hydrogen as the reference species. There is no hydrogen in the feed. Hydrogen is produced in reaction (R1) and consumed in reaction (R2). Apply Eq. 1.7c to hydrogen. N_ H2 = N_ H 2 ,F þ ξ_ 1 - ξ_ 2 mol mol _ ) 27 =0 þ ξ1 - ξ_ 2 s s mol ) ξ_ 1 - ξ_ 2 = 27 s Solve the preceding simultaneously in ξ_ 1 and ξ_ 2 equations to obtain the extents of the reactions, R1 and R2. ξ_ 1 = 38 mol=s and ξ_ 2 = 11 mol=s

1.2.4

Mass Balances for Systems with Recycle

The strategy of recycling valuable streams is often used in the chemical industry to maximize production and minimize the raw material costs. The best way to solve mass balance problems involving recycle is to perform mass balances around different “envelopes” [5] as shown in the Figure 1.1. Envelope 1, which encompasses the recycle stream can be used for overall mass balances. Envelope 2 can be used for performing mass balances around the process unit. Mass balances can also be performed at the junction point (1) and at the splitter point (2) (Fig. 1.1). Fig. 1.1 Mass balances for systems with recycle

1.2

Mass Balances

15

Example 1.6 illustrates the procedure used for solving mass balance problems involving recycle stream. Example 1.6 Saturated sodium nitrate solution at 60 °C is cooled to 20 °C in a crystallizer/filter unit to facilitate the production of sodium nitrate crystals. A portion of the liquid removed in the crystallizer/filter unit, also a saturated sodium nitrate solution at 20 ° C, is recycled to join the fresh feed solution. The remaining solution is purged to maintain a proper recycle ratio (RR). The mixture of the fresh feed solution and the recycle stream, which is a saturated solution of sodium nitrate at 40 °C, is fed to the crystallizer/separator unit. The required production rate is 275 kg/hr of sodium nitrate crystals. The solubility data for sodium nitrate in water is given in the following table: Temperature (°C): Solubility (kg NaNO3/100 kg H2O):

60 122

40 102

30 95

20 87

Determine: A. the mass flow rate of the fresh feed required. B. the mass flow rate of the purge solution. C. the recycle ratio, that is, the ratio of the mass flow rate of the recycle stream to the mass flow rate of the fresh feed. D. the mass flow rate of the feed to the crystallizer. Solution Draw the schematic diagram for the process as shown. The following nomenclature is used: S: solution from the crystallizer/filter F: fresh feed C: feed to crystallizer/filter U: purge stream P: product crystals

The sodium nitrate feed going into the boundary is a saturated solution at 60 °C. Calculate the amount of sodium nitrate in the saturated fresh feed solution using the solubility data as shown. Since the solubility of sodium nitrate at 60 °C is 122 kg NaNO3/100 kg water, the mass fraction of sodium nitrate in the saturated fresh feed solution is

16

1

xF =

Mass and Energy Balances

122 kg NaNO3 = 0:5495 kg NaNO3 =kg soln 222 kg Soln

The solution from the filter is a saturated NaNO3 solution at 20 °C. Since the solubility of sodium nitrate at 20 °C is 87 kg NaNO3/100 kg water, the mass fraction of sodium nitrate in the solution from the filtrate is xS =

87 kg NaNO3 = 0:4652 kg NaNO3 =kg soln 187 kg Soln

The preceding value is also the mass fraction of sodium nitrate in the recycle and purge streams. Therefore, xS = xR = xU = 0:4652 kg NaNO3 =kg soln A. Overall mass balance around the red, dashed boundary: Mass flow in = mass flow out m_ F = m_ P þ m_ U ) m_ U = m_ F - m_ P NaNO3 balance around the red, dashed boundary: Mass flow rate of sodium nitrate in = mass flow rate of sodium nitrate out ðm_ F ÞðxF Þ = ðm_ P ÞðxP Þ þ ðm_ U ÞðxU Þ Substitute for m_ U from the overall mass balance equation. ðm_ F ÞðxF Þ = ðm_ P ÞðxP Þ þ ðm_ F - m_ P ÞðxU Þ Substitute all the known values in the preceding equation and solve for m_ F . Note that the units for all mass fraction terms is kg NaNO3/kg solution

ðm_ Þðx - xU Þ m_ F = P P xF - xU

kg NaNO3 ð1 - 0:4652Þ hr = 0:5495 - 0:4652 = 1745 kg=hr 275

1.2

Mass Balances

17

B. Determine m_ U from the overall mass balance equation. m_ U = m_ F - m_ P = 1745

kg kg - 275 = 1470 kg soln:=hr hr hr

C. Mass balances at junction 1, where the fresh feed and recycle streams join, results in the following equations with the nomenclature as shown in the schematic.

Overall mass balance at junction 1: Mass flow in = mass flow out m_ F þ m_ R = m_ C The feed stream to the crystallizer is a saturated NaNO3 solution at 40 °C. Since the solubility of sodium nitrate at 40 °C is 102 kg NaNO3/100 kg water, the mass fraction of sodium nitrate in the recycle solution is xC =

102 kg NaNO3 = 0:5050 kg NaNO3 =kg soln 202 kg Soln

Note that the unit for all mass fraction terms is kg NaNO3/kg solution in the following equations. NaNO3 balance at junction 1: Mass flow rate of sodium nitrate in = mass flow rate of sodium nitrate out

18

1

Mass and Energy Balances

m_ F xF þ m_ R xR = m_ C xC ) ðm_ F Þð0:5495Þ þ ðm_ R Þð0:4652Þ = ðm_ F þ m_ R Þð0:5050Þ Divide each term of the preceding equation by m_ F and substitute the nomenclature RR (recycle ratio) for the term mm__ FR to obtain the following equation (the units for mass fractions have not been shown for purpose of brevity). 0:5495 þ 0:4652RR = ð1 þ RRÞð0:5050Þ ) kg Recycle RR = 1:12 kg fresh feed D. Use the preceding results to calculate the mass flow rate of the feed to the crystallizer m_ C = ð1 þ RRÞ ) m_ F m_ C = ð1 þ RRÞðm_ F Þ kg feed soln: = 3699 kg soln:=hr ) m_ C = ð1 þ 1:12Þ 1745 hr ÷ m_ F

m_ C = m_ F þ m_ R →

Verification of Results: The results obtained can be verified for consistency by using an overall mass balance for the crystallizer/filter as shown and at the split point as shown. Calculate the mass flow rate of the recycle stream by using the recycle ratio of 1.12. kg feed soln: hr = 1954 kg recycle soln:=hr

m_ R = 1:12m_ F = ð1:12Þ 1745

Overall mass balance for the crystallizer/filter: Mass flow in = mass flow out m_ C = m_ P þ m_ S ) m_ S = m_ C - m_ P = 3699

kg kg - 275 = 3424 kg=hr hr hr

Overall mass balance at the split point of the solution from the filter into recycle and purge streams:

1.2

Mass Balances

19

Mass flow in = mass flow out

m_ S = m_ R þ m_ U = 1954

kg kg þ 1470 = 3424 kg=hr hr hr

The same result for m_ S was obtained earlier by using the overall mass balance for the crystallizer/filter proving the consistency of the results obtained. Example 1.7 The reaction of methanol with oxygen in the presence sodium dichromate and sulfuric acid as catalysts results in the formation of formaldehyde as per the following reaction equation: 2CH3 OH þ O2

Na2 Cr2 O7 , H2 SO4

→ 2HCHO þ 2H2 O

The reaction takes place in a reactor, where the single pass conversion of methanol is 60%. The unreacted methanol and oxygen in stoichiometric proportion are separated from the products and recycled to improve the yield of formaldehyde. The molecular weights (lbm/lbmol) are: Methanol = 32, oxygen = 32, formaldehyde = 30, water = 18. In order to produce 900 lbm/hr of formaldehyde, determine: A. the lbm/hr of fresh feed required. B. the recycle ratio, (lbm/hr recycle)/(lbm/hr fresh feed). Solution Draw the schematic diagram for the reactor system as shown.

Subscripts F, P, and R represent the feed, product, and recycle streams respectively.

20

1

Mass and Energy Balances

Basis 900 lbm/hr HCHO(formaldehyde) product. Calculate the molar production rate of formaldehyde by dividing the mass production rate by its molecular weight. 900 lbm m_ hr N_ P,HCHO = P,HCHO = = 30 lbmol=hr lbm M HCHO 30 lbmol As per the stoichiometric equation, Moles H2 O produced = moles HCHO produced ) N_ P,H2 O = 30 lbmol=hr A. Apply the stoichiometric equation for the red, dashed boundary. N_ F,CH3 OH = N_ P,HCHO = 30 lbmol=hr As per the stoichiometric equation on the reactant side, 1 lbmol 1 N_ F,O2 = N_ F,CH3 OH = = 15 lbmol=hr 30 2 hr 2 Convert the molar feed rates of methanol and oxygen to mass flow rates and add them up to get the mass flow rate of the fresh feed as shown. m_ F = m_ F,CH3 OH þ m_ F,O2

= N_ F,CH3 OH ðM CH3 OH Þ þ N_ F,O2 ðM O2 Þ = 30

lbmol hr

lbmol hr = 1440 lbm=hr þ 15

32

lbm lbmol

32

lbm lbmol

B. The feed to the reactor consists of the fresh feed and the recycle stream. Therefore, the molar feed rate of methanol to the reactor is lbmol _ N_ CH3 OH,reactor = N_ F,CH3 OH þ N_ R,CH3 OH = 30 þ N R,CH3 OH hr Since the single pass conversion of methanol in the reactor is 60%, 60% of the molar feed of methanol to the reactor is converted to formaldehyde, which is the molar rate of production of formaldehyde (30 lbmol / hr). Hence,

1.2

Mass Balances

21

0:60 30

lbmol _ lbmol þ N R,CH3 OH = 30 ) hr hr

N_ R,CH3 OH = 20 lbmol=hr Since the recycle stream consists of methanol and oxygen in stoichiometric proportion, 1 lbmol 1 N_ R,O2 = N_ R,CH3 OH = = 10 lbmol=hr 20 2 hr 2 Calculate the mass flow rate of the recycle stream by multiplying the molar flow rates of methanol and oxygen by their respective molecular weights. m_ R = m_ R,CH3 OH þ m_ R,O2 = N_ R,CH3 OH ðM CH3 OH Þ þ N_ R,O2 ðM O2 Þ = 20

lbmol hr

lbmol hr = 960 lbm=hr þ 10

32

lbm lbmol

32

lbm lbmol

Hence the required recycle ratio is 960 lbm m_ R hr = = 0:67 m_ F 1440 lbm hr

1.2.5

Mass Balances for Systems with Bypass

When a process unit is operating as per the set design conditions and there is a change in the operating load, the end result is more than the desired extent of change to the input stream. For example, a process unit designed to remove a certain amount of moisture from air ends up removing more than the desired amount of moisture when the flow rate of air is decreased due to change in process needs. In such situations, rather than re-designing the process unit, it is preferable to split the feed stream and have a portion of the feed stream bypass the process unit. The desired value of the process variable can be achieved by adjusting the fraction of the feed stream bypassing the process unit as illustrated in the examples to follow. Similar to the strategy used in recycle systems, the best way to solve mass balance problems involving bypass streams is to perform mass balances around different “envelopes” [5] as shown in Fig. 1.2. Envelope 1, which encompasses the bypass stream can be used for overall mass balances. Envelope 2 can be used for performing mass balances around the process unit. Mass balances can also be performed at the splitter point (1) and at the junction point (2) (Fig. 1.2).

22

1

Mass and Energy Balances

Fig. 1.2 Mass balances for systems with bypass

Example 1.8 A process dryer consisting of silica gel reduces the moisture content of substances by absorbing a portion of the moisture depending on the amount of gel present and equilibrium characteristics of the absorber. A silica bed absorber is designed to remove 50% of the moisture content of the entering air, which has a moisture content of 0.008 lbm H2O/lbm dry air. However due to changes in process parameters only 40% of the moisture content of the entering air needs to be removed. Calculate the fraction of the entering air that needs to bypass the absorber to meet the new process conditions. Solution Subscript “in” represents the fresh inlet stream, subscript “b” represents the bypass stream, subscript “sbi” represents the inlet stream to the silica bed, subscript “sbo” represents the outlet stream from the silica bed, and subscript “out” represents the final outlet stream. The fresh inlet stream splits into the bypass stream and another stream which goes through the absorber and both the streams will have the same moisture content as the fresh inlet stream. Therefore, ωsbi = ωb = ωin =

0:008 lbm H2 O lbm d a

Since the process requires only 40% of the moisture in the fresh inlet stream to be removed, the final outlet stream should have 60% of the moisture content of fresh inlet stream. Hence, ωout = 0:60

0:008 lbm H2 O = 0:0048 lbm H2 O=lbm d a lbm d a

The silica bed absorber removes 50% of the entering moisture. Therefore, ωsbo = 0:50ωsbi = 0:50

0:008 lbm H2 O = 0:0040 lbm H2 O=lbm d a lbm d a

1.2

Mass Balances

23

ma is the mass flow rate of dry air and it remains constant throughout the process. f represents the fraction of the entering air that bypasses the silica bed absorber. Therefore, the mass flow rate of the bypass stream is f ðm_ a Þ and the mass flow rate of air going through the absorber is ð1- f Þm_ a . Draw the schematic diagram for the process including all the known information and symbols as shown.

Water balance at junction 2 results in the following equations: Water flow into the junction = water flow out of the junction ð1 - f Þm_ a

lbm d a hr

0:004 lbm H2 O lbm d a þ f m_ a

= m_ a

lbm d a hr

lbm d a hr

0:008 lbm H2 O lbm d a

0:0048 lbm H2 O lbm d a

Simplify and solve the preceding equation for f, the fraction of the entering air that bypasses the absorber. The units for moisture content in the following equation is lbm H2O / lbm d a and are not shown for the sake of brevity. ½ð1- f Þ0:004 þ ½f × 0:008 = 0:0048 ) f = 0:20 Thus, 20% of the entering air must bypass the absorber to satisfy the process requirements.

1.2.6

Mass Balances for Non-reactive Unsteady-State Systems

In unsteady-state systems, there is accumulation of mass within the system (control volume). The accumulation term in the general mass balance equation (Eq. 1.1) will have a non-zero value and must be included in the equation. Further, the generation

24

1

Mass and Energy Balances

Fig. 1.3 Mass balance for non-reactive unsteady-state system

and consumption terms are zero for non-reactive systems [5, 6]. Therefore, the mass balance equation for non-reactive unsteady-state systems will be Input - Output = Accumulation

ð1:11Þ

The schematic for unsteady-state systems is shown in Fig. 1.3. The mass within the system is not constant and it varies with time. Apply Eq. 1.11 to the system shown in Fig. 1.3 to obtain the mathematical expression for unsteady-state mass balance with no reaction. m_ in - m_ out =

dm dt

ð1:12Þ

Equation 1.12 is a first order differential equation that can be applied and solved for unsteady-state systems as shown in the following examples and practice problems. Example 1.9 A 500 L supply tank is designed to supply oxygen-enriched air with 40 mole% oxygen. Initially, the tank contains pure air. Pure oxygen is added to the tank at a rate of 0.002 m3/s and the enriched air is withdrawn from the supply tank at the same rate. The air-oxygen mixture is well mixed so that the concentration of oxygen in the outlet stream is the same as the concentration of oxygen in the tank. All streams and the supply tank are at 20 °C and 101 kPa. Determine the time required to achieve a concentration of 40 mole% oxygen in the outlet stream. Solution Draw the schematic diagram for the process.

Calculate the molar flow rate of the gases using the ideal gas law: PV_ = N_ RT

1.2

Mass Balances

25

(Note: More details about the ideal gas law are given in Chap. 2 on Thermodynamics). Convert the temperature to absolute value: T = 20 ° C þ 273 = 293 K From the ideal gas law, PV_ N_ in = N_ out = = RT

ð101 kPaÞ 0:002 8:314

m3 s

kJ ð293 KÞ kmol  K

= 0:000083 kmol=s

Note on Units :

ðkPaÞ kJ kmolK

m3 s

ðKÞ



kN m3 s m2 kNm kmol

 kmol=s

Calculate the moles in the tank also by using the ideal gas law.

N=

PV = RT

ð101 kPaÞ 500 L ×

1 m3 1000 L

kJ ð293 KÞ kmol  K = 0:0207 kmol 8:314

The mole fraction of oxygen in the tank,yO2 , which is the same as the mole fraction of oxygen in the outlet stream, keeps changing with time. Hence, this is an unsteady-state problem. Derive the expressions for the molar flow rate of oxygen and the moles of oxygen. Since pure oxygen is fed to the tank, N_ O2 ,in = N_ in yO2 ,in = N_ in × 1 = N_ in = 0:000083 kmol=s Similarly, N_ O2 ,out = N_ out yO2 ,out = 0:000083yO2 ,out kmol=s N O2 = NyO2 = 0:0207yO2 kmol O2 Apply the unsteady-state mass balance equation (Eq. 1.12) for oxygen. Moles can be used instead of mass since moles = mass/mol. wt. and mol. wt. cancels throughout the equation.

26

1

Mass and Energy Balances

dN O2 N_ O2 ,in - N_ O2 ,out = dt Substitute the known values derived earlier into the preceding equation (each term has the units kmol O2/s and is not shown for the sake of brevity). d 0:0207yO2 ) dt dy dy = ð0:0207Þ O2 ) 0:004 1 - yO2 = O2 dt dt

0:000083 - 0:000083yO2 = 0:000083 1 - yO2

The preceding equation is a first order differential equation in mole fraction of O2. Separate the variables integrated with the limits, when t = 0, yO2 = 0 and when t = t, yO2 = 0:40 t

0:40

dt =

0:004 0

0

dyO2 1 - yO 2 0:40

) 0:004t = - ln 1 - yO2 0 ) 0:004t = lnð1 - 0Þ - lnð1 - 0:40Þ - lnð0:60Þ ) t= = 127:71 s 0:004 Note: While evaluating the integral on the right-hand side of the preceding equation, the limits are switched due to the negative sign that precedes natural log (ln).

1.3

Energy Balances

Similar to mass balances being based on the law of the conservation of mass, energy balances are based on the law of the conservation of energy [6]. The law of conservation of energy is essentially the first law of thermodynamics which states that energy can neither be created nor destroyed (except in the situation of nuclear reactions, where mass is converted to energy). Energy can merely be transformed from one form of energy to another form of energy. For example, automobile engines convert heat energy generated by the combustion of fuel to mechanical kinetic energy enabling the motion of the automobile. Even if all the heat energy is not completely converted to equivalent kinetic energy, it does not imply the disappearance of energy, rather what happens is some energy is transformed into friction losses and heat losses also account for heat energy that is not converted to equivalent work.

1.3

Energy Balances

1.3.1

27

Energy Balance for Closed Systems – First Law of Thermodynamics

A closed system has a constant mass of a substance within the system boundaries, that is, there is no transfer of mass across system boundaries. However, energy in any form can enter or leave the system, that is, energy transfer is possible in a closed system [1]. Consider a fluid being heated in a closed container as a reference system. The mass of the fluid within the container remains constant. However, the fluid is gaining heat energy and some of the energy leaves the system due to heat losses to the surroundings. The situation is graphically depicted in Fig. 1.3. In the preceding figure, Ei and Ef represent the total initial and the total final energy of the system. The total energy is the sum of the internal energy (U ), kinetic energy (KE), and potential energy (PE) of the system. However, the system being considered is a closed, stationary container. Since the system is stationary, there will be no changes in kinetic and potential energies. Hence, the system energy comprises of only the total internal energy of all the fluid molecules in the container and the initial and final states of energy can be represented as Ui and Uf respectively. The first law of thermodynamics for a closed system can be stated as: Net energy added to the system will be equal to the change in system energy, that is the difference between the final and initial energy states of the system. Energy can be added to the system in the form of heat or work performed on the system like compression of a fluid and similarly energy can be removed in the form of heat or work. Apply the first law to the system shown in Fig. 1.3. The net energy added to the system in this case is Qin - Qout. Note that the net energy added to the system can be positive or negative depending on the relative values of Qin and Qout. Since the system is stationary, the change in system energy is ΔU = Uf - Ui. Hence, the application of the first law for the system shown in Fig. 1.3 results in the following equation: ΔU = Qin - Qout Thus, if the heat added is greater than the heat removed, there will be an increase in the internal energy of the system and conversely, if the heat removed is greater than the heat added, there will be a decrease in the internal energy of the system. Consider the situation where energy is removed from the system in the form of work, for example due to addition of heat, an expanding gas in a piston-cylinder Fig. 1.3 Energy transformations for a closed thermodynamic system

28

1

Mass and Energy Balances

Fig. 1.4 Energy balance (first law of thermodynamics) for closed, stationary systems

arrangement will push the piston resulting in work being performed by the system. A common representation of the first law for a stationary closed system is: Q - W = ΔU

ð1:13Þ

The schematic diagram for the system based on Eq. 1.13 is shown in Fig.1.4. In Eq. 1.13, Q is the heat transfer to the system and W is the work from the system. Q is positive if heat is added to the system and Q is negative if heat is removed from the system.W is positive if work is performed by the system and W is negative if work is performed on the system. There are chances of obtaining erroneous results if incorrect signs are assigned to Q and W. Therefore, the best strategy to solve problems involving closed systems is to draw the schematic diagram for the situation described with proper direction of arrows for heat transfer and work. For example, if heat is added to the system, then the arrow associated with Q should be pointing inward to the system. Similarly, when work is performed by the system, the system loses energy and the arrow associated with W should be pointing outward from the system. After drawing an appropriate and an accurate schematic, apply the following equation to the system represented in the schematic. Net energy added to the system = Energy added - Energy removed = ΔU ð1:14Þ Example 1.10 A piston-cylinder arrangement contains 100 L steam at 101 kPa and 150 °C. Due to expansion of steam at constant pressure, the volume of steam increases to 112 L. The relevant properties of steam can be obtained from steam tables but are provided here for reference. At 101 kPa and 150 °C: specific enthalpy, h = 2776 kJ/kg specific volume, v = 1.9364 m3/kg Determine: A. the final temperature of steam. B. the work of expansion performed by steam. C. the change in internal energy of steam using the first law.

1.3

Energy Balances

29

Solution Note: Please refer to Chap. 2 on Thermodynamics for definitions of thermodynamic properties, steam tables, and details on thermodynamic processes. A. Calculate the mass of steam using the initial volume and initial specific volume. 3

m=

1m V 1 100 L × 1000 L = = 0:0516 kg 3 v1 1:9364 mkg

Since this is a closed system, the mass of steam in the cylinder remains constant. Calculate the final specific volume of steam using the calculated mass and the final volume. 3

1m 112 L × 1000 m3 V L v2 = 2 = = 2:1705 m 0:0516 kg kg

From superheated steam tables, determine the temperature corresponding to this calculated value of specific volume (v2 = 2.1705 m3/kg). An excerpt of the steam table is provided here for reference. Properties of superheated steam at 101 kPa Temperature (°C) 100 v (m3/kg) 1.6958 h (kJ/kg) 2676 s (kJ/kg) 7.3614

150 1.9364 2776 7.6134

200 2.1720 2875 7.8343

From the preceding table, when the specific volume, v2 = 2.1705 m3/kg the corresponding temperature is T2= 200 °C B. For a constant pressure expansion of a gas, the work done is the product of the pressure times change in volume.

W = PΔV = PðV 2 - V 1 Þ = ð101 kPaÞ

112 L ×

1 m3 1000 L - 100 L ×

= 1:212 kJ

Note : kPa × m3 

kN × m3  kN  m  kJ m2

1 m3 1000 L

30

1

Mass and Energy Balances

C. Apply the first law of thermodynamics for a closed system (Eq. 1.14) to the schematic shown here. The heat transfer is the total change in enthalpy for a closed system operating at constant pressure.

Net energy added to the system = Energy added - Energy removed = ΔU Q - W = ΔU ) ΔU = mΔh - W = ð0:0516 kgÞ 2875

kJ kJ - 1:212 kJ - 2776 kg kg

= 3:896 kJ

1.3.2

Fundamental Energy Balance Equation

Similar to the general mass balance equation, the general energy balance equation can be written using an open system control volume (CV) as a reference [5]. In open systems, due to the flow of each species through the CV, the flow work must be added to the internal energy of each species. Since the specific enthalpy is h = u + pv, where the “pv” term represents the flow work, the energy associated per unit mass of each species is the enthalpy of the species. Hence, it is imperative to use specific enthalpies for energy balances in open systems. In closed systems, specific internal energy is used, while in open systems, specific enthalpies are used. The fundamental energy balance equation applied to an open system, an entity with well-defined boundaries (CV), is as follows. Input þ Generation─Output─Consumption = Accumulation Each term in Eq. 1.15 is explained here.

ð1:15Þ

1.3

Energy Balances

31

Input refers to the input energy (enthalpy) flow rates of species entering through the system boundaries. Generation refers to the rate of production of energy within the system boundaries. Usually, energy is generated due to exothermic chemical reaction(s). Output refers to the output energy (enthalpy) flow rates of species leaving through the system boundaries. Consumption refers to the rate of decrease of energy within the system boundaries. Generally, energy is consumed due to endothermic chemical reaction(s). Accumulation is the rate at which energy increases or decreases within system boundaries. Using the preceding explanations and using subscript i for input streams and subscript e for exit streams, Eq. 1.15 can be mathematically expressed as shown in Eq. 1.16. n

E_ ji þ E_ gen:,CV -

j=1

1.3.2.1

n

dE E_ je - E_ cons:,CV = dt j=1

ð1:16Þ CV

Steady-State Energy Balance

At steady state, there is no accumulation of energy within the control volume and dE dt CV = 0. Apply the energy balance equation at steady state to the system shown in Fig. 1.5 and include the kinetic and potential energy terms for the sake of generality. _ is the shaft power output from the Q_ is the rate of heat transfer into the system and W system. Rate of energy entering the CV = Rate of energy out of leaving the CV. n

m_ ji hji þ j=1

v2ji þ gzji 2

þ Q_ =

n

m_ je hje þ j=1

v2je þ gzje 2

The preceding equation is more commonly represented as

Fig. 1.5 Open system control volume (CV) for energy balance

_ þW

32

1 n

m_ je hje þ j=1

v2je þ gzje 2

n

m_ ji hji þ

j=1

Mass and Energy Balances

v2ji _ þ gzji = Q_ - W 2

ð1:17Þ

Equation 1.17 is commonly known as the Steady Flow Energy Equation (SFEE) [5]. In most chemical engineering applications, the kinetic and potential energy terms are insignificant (compared to the enthalpy terms) or zero and in addition, there is no shaft work produced. With the preceding assumptions, Eq. 1.17 simplifies to the following equation commonly used in chemical engineering energy balances [5, 6]. n

n

m_ je hje j=1

m_ ji hji = Q_

ð1:18Þ

j=1

In Eq. 1.18,Q_ is the rate of heat transfer, which is positive if added to the system and negative if removed from the system. Example 1.11 Liquid water and steam are mixed in an open feedwater heater (FWH) to form the feedwater to a boiler. Stream 1 is 900 lbm/min of water at 80° F from a condenser and stream 2 is 300 lbm/min of saturated steam at 300 psia after one stage of expansion in a turbine. The boiler produces superheated steam at 500 psia and 700° F. Enthalpy data for steam can be obtained from online resources or steam tables but is provided here for reference and use. Specific enthalpy, h is in Btu/lbm. ∘

∘

hf ð80 FÞ = 48:1, hg ð300 psiaÞ = 1204, h ð500 psia, 700 FÞ = 1357 Calculate the required rate of input to the boiler neglecting kinetic energy changes. Solution Draw a schematic diagram representing the feedwater heater and all the associated streams.

Designate the given enthalpies to the appropriate streams.

1.3

Energy Balances

33 ∘

hF1 = hf ð80 FÞ = 48:1 Btu=lbm hF2 = hg ð300 psiaÞ = 1204 Btu=lbm ∘

hs = hð500 psia, 700 FÞ = 1357 Btu=lbm Calculate the mass flow rate and enthalpy of the boiler feed stream using fundamental mass balance and fundamental energy balance equations for the FWH. Mass balance: Mass flow into FWH = mass flow out of FWH F1 þ F2 = FB ) lbm lbm F B = 900 þ 300 = 1200 lbm= min min min Energy balance: Energy flow into FWH = energy flow out of FWH F 1 hF1 þ F 2 hF2 hB =

F 1 hF1 þ F 2 hF2 FB

= F B hB ) lbm Btu lbm Btu 900 þ 300 × 48:1 × 1204 min lbm min lbm = lbm 1200 min = 337:1 Btu=lbm

Calculate the rate of heat input to the boiler using energy balance for the boiler. Rate of energy into boiler = rate of energy out of boiler F B hB þ Q_ B = Shs ) Q_ B = Shs - F B hB

lbm Btu lbm Btu × 337:1 × 1357 - 1200 min lbm min lbm = 1:224 × 106 Btu= min = 1200

Example 1.12 A feed stream of 40% (by mass) sodium hydroxide solution at 80° F enters a wellinsulated mixing vessel at a mass flow rate of 5 lbm/min. It is desired to produce a dilute sodium hydroxide solution at a temperature of 120° F after complete mixing of the feed sodium hydroxide solution with pure water at 160 °F. Calculate the mass flow rate of water required for the adiabatic dilution process using the enthalpy concentration diagram provided here.

34

1

Mass and Energy Balances

Solution Draw the schematic diagram for the process.

Mass balance for the mixer results in the following equation. P = F1 þ F2: From the enthalpy-concentration diagram, enthalpy of stream 1 (40% NaOH, 80 °F) is h1 = 75 Btu/lbm enthalpy of stream 2 (Water, 160 °F) is h2 = 125 Btu/lbm

Since this is an adiabatic mixing process, there is no heat transfer (no heat gain or heat loss) by/from mixer. Energy balance for the mixer results in the following equation.

1.3

Energy Balances

35

F 1 h1 þ F 2 h2 = PhP = ðF 1 þ F 2 ÞhP Substitute the known values into the preceding equation. 5

lbm Btu Btu lbm × 75 þ F 2 × 125 = 5 þ F 2 hP min lbm lbm min

Simplify the preceding equation. 375

Btu lbm þ 125F 2 = 5 þ F 2 hP min min

Determine the mass fraction of NaOH in the product solution. lbm NaOH 5 lbm soln × 0:4 mNaOH lbm soln = xNaOH,P = mNaOH þ mwater 2 lbm NaOH þ ð3 þ F 2 Þlbm water 2 = 5 þ F2 The temperature of the product solution is given as 120° F. Use the following procedure to solve for F2 by trial and error. 1. Assume a mass fraction for NaOH in the product solution. 2. Determine the enthalpy, hP, from the enthalpy-concentration chart corresponding to the assumed value of the mass fraction and 120° F. 3. Determine F2 using the preceding equation for mass fraction. 4. Substitute the values for F2 and hP in the simplified energy balance equation and check if the equation is satisfied. Pure water is being mixed with a stream containing 0.40 mass fraction of NaOH. Therefore, the product stream will have 0 < xNaOH < 0.40. Reviewing the information provided on the temperatures of the streams (80° F, 0.4 NaOH solution with xNaOH = 0.40, 160° F pure water and 120° F product solution), it is reasonable to assume a mass fraction of = 0.34 for the product solution. From the enthalpyconcentration diagram for sodium hydroxide solution, determine the enthalpy at xNaOH, P = 0.34 and 120° F )hP = 82 Btu/lbm soln. Determine F2 using the equation for mass fraction. ) F2 =

2 - 5 × 0:34 = 0:88 lbm= min: 0:34

Substitute for F2 on the left-hand side of the simplified energy balance equation, and the left-hand side will be equal to

36

1

375

Mass and Energy Balances

Btu þ 125F 2 = 375 þ 125 × 0:88 = 485 Btu= min min

Substitute for F2 and hP on the right-hand side of the simplified energy balance equation, right-hand side will be equal to 5

lbm lbm lbm þ 0:88 þ F 2 hP = 5 min min min

82

Btu = 482:2 Btu=lbm lbm

Since the values of the left-hand side and right-hand side of the energy balance equation agree with each other, the assumed value of mass fraction of sodium hydroxide in the product solution is correct. Therefore, the required flow rate of the diluting water at 160° F is F 2 = 0:88 lbm= min Example 1.13 10 lbmol/min of propane (C3H8) gas at 200 psia and 100 °F is to be heated to 450 °F in a well-insulated heat exchanger using superheated steam at 580 °F and 80 psia as the heating medium, with steam leaving the heat exchanger as a saturated liquid at 80 psia. Using the data given, determine: A. Btu/min heat required B. lbm/min steam required Data: M C3 H8 = 44 lbm=lbmol, M H2 O = 18 lbm=lbmol cp,C3 H8 ,avg = 23:33 Btu=lbmol‐ ° R Steam enthalpies (Btu/lbm): hv ð80 psia, 580 ° FÞ = 1321, hf ð80 psiaÞ = 282:2 Solution A. Calculate the rate of heating required by using the molar flow rate and molarspecific heat of propane. Q_ = N_ C3 H8 cp,C3 H8 ΔT

lbmol Btu 23:33 ð450 ° F‐100 ° FÞ min lbmol‐ ° R = 81, 655 Btu= min = 10

B. The required rate of heating is supplied by steam. Therefore, from energy balance, Rate of heat gained by propane = rate of heat lost by steam.

1.3

Energy Balances

37

Q_ = m_ s ðhs,in - hs,out Þ ) Q_ m_ s = = hs,in - hs,out

1.3.3

81655 1321

Btu min

Btu Btu - 282:2 lbm lbm

= 78:61 lbm= min

Energy Balances for Reactive Processes

Energy balances on reactive processes involve the use of thermodynamic data such as Heats of formation, Heats of reaction, and Heats of combustion. Standard values of these data are available in literature. The standard conditions are 25 °C and 1 atmosphere pressure, which is a commonly used reference state in enthalpy calculations for reactive processes. Tables, widely available in chemical engineering literature, list standard heats of formation and standard heats of combustion for various compounds. The definitions of thermodynamic data used in energy balance calculations are provided in the following section [5, 6]. Standard conditions of 25 ° C and 101 kPa (77 °F and 14.7 psia) are implied in all the definitions and the symbol H represents molar enthalpy (kJ/kmol, Btu/lbmol) and the superscript “0” associated with molar enthalpies represents the use of standard conditions. Conversion factor: 1 kJ/mol = 430 Btu/lbmol Note: mol  gmol.

1.3.3.1

Definitions of Thermochemical Data 0

Standard Heat of Formation H f : The standard heat of formation of a compound is the heat of the formation reaction from its atomic constituents as they normally occur in nature [7]. The standard heat of formation of an element is Zero. 0

Standard Heat of Reaction H r : The standard heat of reaction is the heat generated or consumed due to the reaction and it can be calculated from the standard heats of formation using the following equation.

0

0

Hr =

Hf i

0

-

i products

Hf i

ð1:19Þ

i reactants

0

When the enthalpy of products is higher than the enthalpy of reactants, H r is positive and the reaction absorbs heat. Such reactions are called endothermic reactions. When the enthalpy of reactants is greater than the enthalpy of products,

38

1

Mass and Energy Balances

0

H r is negative and the reaction releases heat. Such reactions are called exothermic reactions. Example 1.14 It is desired to produce 1008 kg/hr of ethylene (C2H4) by the dehydrogenation of ethane (C2H6) to form ethylene and hydrogen (H2) in stoichiometric proportion. The standard heats of formation are: 0

H f ,C2 H6 = - 84:4 kJ=mol 0

H f ,C2 H4 = 52:4 kJ=mol: The molecular weights are (kg/kmol): C2 H6 = 30 and C2 H4 = 28 Calculate the required rate of heat transfer from/to the reactor in kW, assuming ideal conditions of complete conversion of ethane to ethylene and both reactants and products are at standard conditions of 25 °C and 1 atm. Solution The reaction equation is C2 H6 → C2 H4 þ H2 Draw the schematic diagram for the dehydrogenation reactor.

Basis: 1008 kg/hr of ethylene produced Calculate the molar production rate of ethylene. 1008 kg m_ hr = 36 kmol=hr N_ C2 H4 = C2 H4 = kg M C2 H 4 28 kmol Calculate the standard heat of reaction per mol (gmole) of ethane reacting using Eq. 1.19.

1.3

Energy Balances

0

Hr =

39

0

i

Hf

0

-

i

i

prod:

Hf

0

0

0

= H f ,C2 H4 þ H f ,H2 - H f ,C2 H6

i react:

kJ kJ þ0 mol mol = 136:8 kJ=mol = 52:4

- 84:4

kJ mol

Since the standard heat of reaction is positive, the reaction is endothermic (absorbs heat) requiring the supply of heat. Calculate the rate of heat supply required for ethane flow rate of 36 kmol/hr. kmol Q_ in = 36 hr = 1368 kW

136:8

kJ 1000 mol × mol kmol

1 hr 3600 s

Note: kW  kJ/s 0

Standard Heat of Combustion H c : The standard heat of combustion of a compound is the heat of reaction of that compound with oxygen to yield specified combustion products. Both the reactants and products are assumed to be at standard conditions of 25 °C and 1 atm. pressure. Thus, if water is a product of combustion, it will be in liquid state at a temperature of 25 °C. The standard heat of reaction of a reaction can be calculated from the standard heats of combustion using the following equation.

0

0

Hr = i

1.3.3.2

0

-

H ci reactants

ð1:20Þ

H ci i

products

Hess’s Law

Hess’s law is very useful in obtaining the desired heat of reaction by algebraic manipulation of available standard heats of reaction, heats of formation, and heats of combustion data [2]. Hess’s law states that if the stoichiometric equation for a desired reaction can be obtained by algebraically manipulating the stoichiometric equations for other reactions whose heats of reactions are available, then the heat of reaction for the desired reaction can be obtained by performing the same algebraic manipulations of the available heats of reactions for the other reactions.

40

1

Mass and Energy Balances

Example 1.15 The following information is available related to the combustion of propane: 0

C3 H8 þ 5O2 → 3CO2 þ 4H2 O ðliq:Þ, H r = - 2219:5 kJ=mol The enthalpy of vaporization of water at standard conditions is 44 kJ/mol. Determine the actual heating value of propane in Btu per standard cubic feet. Solution Write the given combustion equation and the equation for the vaporization of water and manipulate them to obtain the actual heating value when the product water is in the vapor phase.

Convert the actual heating value to Btu/lbmol. AHV = - 2043:5 kJ=mol ×

430 Btu=lbmol = - 878705 Btu=lbmol 1 kJ=mol

Calculate the standard cubic feet per lbmol of gas, using the principles based on the ideal gas law: 1 lbmol of any gas at 32 °F and 14.7 psia occupies 359 ft3. The volume of an ideal gas is directly proportional to the absolute temperature. As per industry standards, standard cubic feet is the volume measured at 60 °F and 14.7 psia. (Note: Please refer to Sect. 2.21 in the thermodynamics chapter for more details on calculations involving the ideal gas law.) AHV Btu 1 lbmol 492 ° R × = - 878705 × = - 2316 Btu=SCF SCF lbmol 359 ft3 520 ° R

1.3.3.3

General Procedure for Energy Balances for Reactive Processes

The procedure to be used for performing energy balances for reactive systems depends on the nature of the problem and the data provided. The key is to choose appropriate reference states for the species involved in the reactions and consistently

1.3

Energy Balances

41

use the same reference states as the basis for calculating enthalpy changes. In most cases, the following general procedure can be used in performing energy balances for reactive processes [5]. 1. A schematic diagram for the process is drawn including all the known information about inlet reactant species. Algebraic symbols are assigned to unknown information about outlet product species. Some of the unknown information can be calculated by material balances for species. Appropriate directions must be used for the arrows representing the flow of species and for the flow of heat (arrows pointing inward to the control volume for inlet streams and arrows pointing outward from the control volume for outlet streams). 2. The reference states for reactants and products are chosen as the elements that constitute the reactants and products at 25 °C (77 °F), keeping in mind that the standard heat of formation of an element is zero. 3. The molar enthalpy,H i , for each reactant and product species is calculated as the sum of the heat of formation of the species at 25 °C and any sensible and latent heats required to bring each species to its inlet or outlet state. 4. A table of molar flow rates, N_ i’s, and the corresponding molar enthalpies, H i’s for all the components is constructed and the summation of the rows is used in the fundamental energy balance equation applied to the control volume in the schematic to calculate the unknown quantities. Rate of energy in = Rate of energy out The heat of reaction is implicitly included in the calculation when the preceding equation is used. 5. Alternately, if the standard heat of reaction is known, the reference state is taken as elements and compounds at standard state (1 atm. and 25 °C/77 °F) and the rate of heat transfer to the reactor can be calculated using the following equation: _ 0þ Q_ = ΔH_ = ξH R

N_ Pi H Pi i

N_ Fi H Fi

ð1:21Þ

i

The nomenclature used in Eq. 1.21 is _ the rate of heat transfer to the reactor (negative sign for Q_ implies heat removal Q: from the reactor) _ the rate of enthalpy change for the process ΔH: _ the extent of the reaction, which can be calculated using Eq. 1.7a, shown here for ξ: _ _ reference ξ_ = N i ±-νNi iF (1.7a) 0

H R : standard heat of reaction N_ Pi : molar flow rate of product species i

42

1

Mass and Energy Balances

H Pi : enthalpy of product species i N_ Fi : molar flow rate of species i in the feed H Fi : enthalpy of species i in the feed νi: Stoichiometric coefficient of species i Example 1.16 The material balance information available from a dehydrogenation reactor is illustrated in the given schematic diagram along with the feed and product temperatures. Additional thermochemical data is: Standard heats of formation (kJ / mol): 0

H f ,C2 H6 = - 84:2,

0

H f ,C2 H4 = 52:4,

0

H f ,CH4 = - 74:63

Average molar heat capacities (J/molK): cp,C2 H6 = cp,H2 =

94:19, cp,C2 H4 = 74:25, cp,CH4 = 53:94, 29:38, cp,N2 = 30:43

Determine the rate of heat input to the reactor. Solution Determine the enthalpy of each reactant and product species using the elements at standard state (1 atm. pressure and 25 °C) as the reference state and by using the given thermochemical data for the species. Note that the standard heat of formation of an element at its standard state is 0. H F,C2 H6 = H 0f ,C2 H6 þ cp,C2 H6 ðT F - T ref Þ kJ = - 84:2 þ mol = - 48:88 kJ=mol

J mol  K ð400 ° C - 25 ° CÞ 1000 J kJ

94:19

1.3

Energy Balances

H P,C2 H6

43

= H 0f ,C2 H6 þ cp,C2 H6 ðT P - T ref Þ kJ = - 84:2 þ mol = - 20:62 kJ=mol

J mol  K ð700 ° C - 25 ° CÞ 1000 J kJ

94:19

44

1

Mass and Energy Balances

H P,C2 H4 = H 0f ,C2 H4 þ cp,C2 H4 ðT P - T ref Þ kJ = 52:4 þ mol

J mol  K ð700 ° C - 25 ° CÞ 1000 J kJ

74:25

= 102:52 kJ=mol

H P,CH4 = H 0f ,CH4 þ cp,CH4 ðT P - T ref Þ kJ = - 74:63 þ mol

J mol  K ð700 ° C - 25 ° CÞ 1000 J kJ

53:94

= - 38:22 kJ=mol The following table can be generated using the preceding information and the material balance data given. Reference state: Elements of feed and product species at 25 °C and 1 atm. N_ Fi kmol/hr 71.42 7.94 NA NA NA

Species C2H6 N2 C2H4 H2 CH4

N_ i H i i

N_ Pi kmol/hr 21.43 7.94 35.71 21.43 28.57

H Fi kJ/mol -48.88 11.41 NA NA NA

N_ i H i

= - 3400 kJ=hr F

i

H Pi kJ/mol -20.62 20.54 102.52 19.83 -38.22

= 2715 kJ=hr P

Since the reactants are at 400 °C and the products are at 700 °C, the heat transfer is assumed to be into the reactor, as shown in the schematic diagram. Energy balance around the reactor results in the following equations.

1.4

Mass and Energy Balances for Combustion Reactions

45

Energy flow in = Energy flow out

þ Q_ =

N_ i H i i

Q_ =

N_ i H i i

N_ i H i

P

i

N_ i H i i

F

= 2715 F

) P

kJ hr

- 3400

kJ hr

= 6115 kJ=hr The positive sign for Q_ indicates the assumed direction of heat transfer into the reactor is correct. Convert the rate of heat transfer to kW. kJ 1 hr Q_ = 6115 × = 1:6986 kW ≃ 1:70 kW hr 3600 s

1.4 1.4.1

Mass and Energy Balances for Combustion Reactions Introduction

Combustion is the chemical reaction of fuel with oxygen to generate heat. The heat generated can be used for generating steam, for powering vehicles, and for many other similar purposes. The products of combustion can be harmful to the environment. Combustion of fuels is studied with two major objectives in mind – determining the amount of fuel and associated air required in any particular situation and finding the quantities of combustion products to assess environmental impact.

1.4.2

Heating Value of Fuels

The heating value of a fuel is the amount of heat generated per unit mass or per unit mole of fuel [4]. The typical units of heating value are Btu/lbm (or kJ/kg) and Btu/lbmol (or kJ/kmol). However, the heating values are also expressed in terms of Btu/gallon (kJ/m3 or J/L) for liquid fuels and Btu/SCF (standard cubic feet) for gaseous fuels. The heating values of fuels are further categorized into higher heating value, HHV (also known as gross heating value) and lower heating value (also known as net heating value (NHV) or actual heating value (AHV)). HHV is the same

46

1 Mass and Energy Balances

as the standard heat of combustion (please refer to Sect. 1.3.3.1). The relationship between HHV and LHV is given by Eq. 1.22. LHV = HHV - mwater hfg

ð1:22Þ

In Eq. 1.22, mwater is the mass of water formed due to combustion and hfg is the enthalpy of vaporization of water at 25 °C or 77 °F (1049 Btu/lbm or 2442 kJ/kg). The basis for Eq. 1.22 is the heat required for vaporizing water (which is not considered in HHV) comes from the combustion of the fuel itself, thus lowering the actual heating value available from the fuel. Example 1.17 A gaseous fuel has the following mass composition: carbon = 24% and hydrogen (H2) = 66% and the rest being non-combustible components including moisture. The standard heats of combustion are: ΔH 0c,C = 14, 100 Btu=lbm ΔH 0c,H2 = 60, 850 Btu=lbm The density of the gaseous fuel is 0.068 lbm/ft3. Calculate the net heating value of the gaseous fuel per as delivered cubic feet. Solution Basis: 1 lbm fuel ) 0.24 lbm C + 0.66 lbm H2 Calculate the higher heating value (HHV) of the fuel using the given standard heat of combustion data. HHV = ðmC Þ ΔH 0c,C þ ðmH2 Þ ΔH 0c,H2 = 0:24

lbm C lbm fuel

14100

Btu lbm C

Btu lbm H2 60850 lbm fuel lbm H2 = 43545 Btu=lbm fuel þ 0:66

Calculate the mass of water vapor formed per pound mass of fuel by using the stoichiometric equation for the combustion of hydrogen. 1 H2 þ O2 → H2 O ) 2 lbm H2 produces 18 lbm H2 O: 2 Therefore,

1.4

Mass and Energy Balances for Combustion Reactions

47

lbm H2 O produced 18 lbm H2 O produced 0:66 lbm H2 = lbm fuel lbm fuel 2 lbm H2 = 5:94 lbm H2 O produced=lbm fuel Calculate the lower heating value (LHV) of the coal using Eq. 1.22 and the enthalpy of vaporization of water. hfg = 1049 Btu=lbm H2 O: LHV = HHV - mwater hfg 5:94 lbm H2 O produced Btu lbm fuel lbm fuel = 37314 Btu=lbm fuel

= 43545

1049 Btu lbm H2 O

Calculate the net heating value (NHV, the same as LHV) of the fuel by using the density of the fuel. NHV = 37314

1.4.3

Btu lbm fuel

0:068

lbm = 2537 Btu=ft3 fuel ft3

Stoichiometry of Combustion Reactions, Theoretical and Excess Air

A balanced stoichiometric reaction for combustion of any fuel can be written provided the molecular or mass composition of the fuel is known [1, 5, 6]. Typically, combustion reactions involve hydrocarbon compounds. During combustion of hydrocarbon compounds, carbon is oxidized to carbon dioxide (assuming complete combustion, partial combustion results in the formation of carbon monoxide) and hydrogen is oxidized to water. The source of oxygen is air, and the composition of air is 21 mole% oxygen and 79% nitrogen resulting in the following expressions: moles N2 = 3:76 × moles O2

ð1:23Þ

moles air = 4:76 × moles O2

ð1:24Þ

However, since nitrogen is inert, it does not participate in the combustion reaction. The moles and mass of nitrogen will be the same on either side of the

48

1 Mass and Energy Balances

combustion equation and the combustion product gases will consist of significant amount of inert nitrogen. A balanced stoichiometric equation can be derived by balancing the elements in the following order to avoid redundancy and too many unknowns – first carbon, next hydrogen, and finally oxygen. Once the stoichiometric coefficient of oxygen (O2) has been established, the stoichiometric coefficient of nitrogen (N2) will be 3.76 times the coefficient of oxygen on the reactant side as well as on the product side due to the inert nature of nitrogen. The moles of air required as per the stoichiometric combustion equation can be obtained by adding up the moles of oxygen and nitrogen on the reactant side. The moles of air thus obtained is called theoretical air, sometimes also termed as stoichiometric air. However, air is supplied in excess of theoretical requirement to ensure sufficient air for complete combustion of carbon to carbon dioxide. The amount of air supplied in excess of the theoretical requirement is called excess air. Typical amounts of excess air used are 10% for gaseous fuels, 20% for liquid fuels, and up to 60% for solid fuels. However, when excess air is used, only the theoretical amount of oxygen required for complete combustion of the fuel (as per the stoichiometric equation) will be used. The remaining unused oxygen will emerge along with the products of combustion. Combustion equations involving excess air must include excess oxygen on the product side of the combustion equation along with the corresponding moles of nitrogen associated with the oxygen supplied. Example 1.18 Butane (C4H10) undergoes combustion with 10% excess air. Determine: A. the theoretical combustion equation. B. the actual combustion equation. C. the air-fuel ratio on mass basis. Solution The theoretical combustion equation with unknown stoichiometric coefficients is: C4 H10 þ aO2 þ bN2 → cCO2 þ dH2 O þ eN2 Balance the atoms for each species on both sides of the reaction equation in the order suggested, that is, first carbon, next hydrogen, and finally Hydrogen. C balance: c = 4 H balance: 2d = 10, therefore, d = 5 O balance: 2a = 2c + d = 2(4) + 5 = 13, therefore, a = 6.5 N2 = 3.76O2) b = 3.76a = 3.76(6.5) = 24.44 = e (a) Theoretical combustion equation is: C4 H10 þ 6:5O2 þ 24:44N2 → 4CO2 þ 5H2 O þ 24:44N2

1.4

Mass and Energy Balances for Combustion Reactions

49

(b) The actual combustion equation for 10% excess air: The coefficients of oxygen and nitrogen in the reactants are multiplied by 1.1 (1.0 for theoretical air and 0.1 for 10% excess air). Excess oxygen is included in the products. C4 H10 þ ð1:1Þ6:5O2 þð1:1Þ24:44N2 → 4CO2 þ 5H2 O þ 24:44N2 þ ð0:1Þ6:5O2 C4 H10 þ 7:15O2 þ26:88N2 → 4CO2 þ 5H2 O þ 26:88N2 þ 0:65O2

(c)

r a=f

1.4.4

29 lbm ð7:15 þ 26:88Þ lbmol air × lb air lb mol = = 58 lb lb fuel mass 1 lbmol fuel × lbmol lb air = 17:02 lb fuel

Analysis of Combustion Products – Flue Gas Analysis

Combustion of hydrocarbon fuels typically results in the following products: carbon dioxide, water vapor, nitrogen associated with air, any excess oxygen, and occasionally small quantities of carbon monoxide in case there is partial combustion. If the fuel has sulfur, sulfur dioxide will also be part of the combustion products. Analysis of combustion products provides information needed for assessing the environmental impact due to the combustion products [5, 6]. The combustion products form a mixture of ideal gases. For an ideal gas mixture, mole fractions, volume fractions, and pressure fractions of components are equal to each other. In the combustion equation, the stoichiometric coefficients of the species represent the mols of the species. Thus, the individual mols of each product, the total mols of the combustion gases, and the mole fraction of each component can be obtained from the combustion equation. The products of combustion are also known as flue gases or stack gases. Orsat Analysis is a method used in determining the volume composition (same as molar composition for ideal gases) of combustion products on a dry basis [3]. However, water vapor is a product of combustion and must be included on the product side of the combustion reaction equation. Using the results of Orsat analysis, a skeleton combustion reaction is written using algebraic symbols for unknown stoichiometric coefficients and unknown molecular formula of the fuel. The values of the unknown variables are obtained using atomic balances for elements as illustrated in the following example.

50

1

Mass and Energy Balances

Example 1.19 The Orsat Analysis of combustion of an unknown liquid hydrocarbon fuel provides the following result on a volume basis: carbon dioxide 8.6%, carbon monoxide 0.9%, oxygen 8.1%, and the rest is nitrogen. Determine the percent excess air used. Solution A. The mole fraction of a component is the same as volume fraction in a mixture of ideal gases. Based on the results of Orsat analysis, the volume percent and hence the mole percent of nitrogen in the combustion product is 100 - 8:6 - 0:9 - 8:1 = 82:4%: Since water vapor is a product of combustion, it must be included in the combustion equation. Write the actual combustion equation based on 100 moles of combustion products using the data from the Orsat analysis and by using algebraic symbols for unknown entities. Ca Hb þ cO2 þ dN2 → 8:6CO2 þ 0:9CO þ eH2 O þ 8:1O2 þ 82:4N2 Obtain the unknown stoichiometric coefficients by using balances for various species in the combustion equation. N2 balance: d = 82.4 d N2 to O2 ratio in air: d = 3:76c ) c = 3:76 = 82:4 3:76 = 21:91 C balance: a = 8.6 + 0.9 = 9.5 O balance: 2c=2ð8:6 þ 8:1Þ þ e þ 0:9=e þ 34:3 ) 2ð21:91Þ =e þ 34:3 ) e=9:52 H balance: b = 2e = 2(9.52) = 19.04 Substitute the values for the unknowns and write the actual combustion equation. C9:5 H19:04 þ 21:91O2 þ82:4N2 → 8:6CO2 þ 0:9CO þ 9:52H2 O þ 8:1O2 þ 82:4N2 B. Theoretical combustion implies complete combustion of all carbon to carbon dioxide and carbon monoxide is not formed and excess oxygen will not be part of combustion products. Derive the theoretical combustion equation as shown here. C9:5 H19:04 þ aO2 þ bN2 → cCO2 þ dH2 O þ bN2 C balance: c = 9.5 H balance: 2d = 19:04 ) d =

19:04 2

= 9:52

1.4

Mass and Energy Balances for Combustion Reactions

51

9:52 O balance: 2a = 2c þ d ) a = 2ð9:5Þþ = 14:26 2 N2 to O2 ratio in air: b = 3.76a = 3.76 × 14.26 = 53.62

Therefore, the theoretical combustion equation is C9:5 H19:04 þ 14:16O2 þ 53:62N2 → 9:5CO2 þ 9:52H2 O þ 53:62N2 For reference, the actual combustion equation obtained from the solution to part A is C9:5 H 19:04 þ 21:91O2 þ82:4N 2 → 8:6CO2 þ 0:9CO þ 9:52H 2 O þ 8:1O2 þ 82:4N 2 Percent theoretical air =

actual moles of air theoretical moles of air

100 =

21:91þ82:4 14:16þ53:62

100 = 153:9%

Hence % excess air used =153.4 % - 100 % = 53.4%

1.4.5

Dew Point of Combustion Products

Water vapor is a product of combustion and condensation of water vapor can ultimately lead to the damage of the stack system due to the formation of corrosive solutions. Water condensation in the stack system can be prevented by maintaining the temperature in the stack system well above the dew point of the combustion products. The partial pressure of water vapor in the combustion gases represents the saturation pressure of water vapor in the mixture. The dew point of the mixture of combustion gases is the saturation temperature at the saturation (partial) pressure of water vapor in the product gas mixture.

1.4.6

Combustion of Coal – Use of Gravimetric Analysis of Coal

The constituents of coal are reported on the basis of mass percentage or mass fraction, also known as gravimetric analysis. Since the combustion reaction occurs on a molar basis, the constituents of coal must be converted to moles in order to determine the combustion equation. The free moisture associated with coal will also be part of the water vapor in the combustion product. Example 1.20 A type of coal has the following gravimetric analysis: carbon 72%, hydrogen 8%, oxygen 4%, nitrogen 2%, free moisture 9%, and the rest is ash. The standard heats of formation are:

52

1

CO2

= - 393:5 kJ=mol

H2 OðvÞ

= - 241:8 kJ=mol

hf hf

Mass and Energy Balances

Determine: A. the SCMH (standard cubic meter per hour) of stack gas produced due to the combustion of 1000 kg/hr of coal. B. the actual heating value of coal (kJ/kg). C. the supply rate of coal required (kg/hr) per MW of power produced if the overall plant efficiency is 42% and the boiler efficiency is 92%.

Solution A. Basis: 1 kg of coal Determine the moles of combustion products as shown in the following table. Species C O2,coal N2,coal H2 H2O, free N2,aira

Mass (kg) 0.72 0.04 0.02 0.08 0.09

Mol. wt. (kg/kmol) 12 32 28 2 18

Moles, m/M (kmol) 0.06 0.00125 0.00071 0.04 0.005

Reaction C+O2→CO2 NA NA H2+½O2→H2O NA

Product CO2 N2 H2O H2O

Total (kmol)

kmol product 0.06 NA 0.00071 0.04 0.005 0.3004a 0.4061

a

Note: The nitrogen in the air supplied as well as in the product gas is calculated as follows. From the reactions shown in the table, the net O2 required based on the combustion reactions of C and H2 and the moles of O2 available in coal is

NO2 = 0:06 þ 0:02 - 0:000125 = 0:0799 kmol The associated nitrogen is NN2 = ð0:0799 kmol O2 Þ

3:76 kmol N2 = 0:3004 kmol N2 kmol O2

Calculate the kmol/hr of stack gas produced when 1000 kg/hr of coal is burned. 0:4061 kmol N_ prod:gas = kg coal

1000 kg coal = 406:1 kmol=hr hr

1.4

Mass and Energy Balances for Combustion Reactions

53

Standard cubic meter per hour (SCMH) is the volume flow rate of a gas measured at 15 °C (288 K) and 101.3 kPa. Calculate the SCMH of the product gas using ideal gas law. kmol 406:1 _ prod:gas RT hr N V_ prod:gas = = P = 9599 SCMH

kJ ð288 KÞ kmol  K 101:3 kPa 8:314

B. Using the results obtained in the solution for part A, write the combustion reaction for 1 kg coal. 0:06C þ 0:04H 2 þ 0:0799O2 þ 0:3004N 2 → 0:06CO2 þ 0:04H 2 O þ 0:3004N 2 Calculate the standard heat of reaction for the combustion reaction using Eq. 1.19 (reproduced here for reference). Note that the standard heats of formation of elements are zero. 0

0

Hr =

Hf i

0

H combn: =

0

i

NiHf

prod:

Hf

ð1:19Þ

i

i

products

reactants

0

-

i

0

-

i

i

NiHf

i react:

40 mol H2 O - 393:5 kJ 60 mol CO2 = þ kg coal mol CO2 kg coal = - 33282 kJ=kg coal

- 241:8 kJ -0 mol H2 O

Note: 1 mol  gmole and 1 kmol  1000 mol. Hence, the kmol of species in the table needs to be multiplied by 1000 to obtain mol of species. The heats of formation are given on the basis of 1 mol. From steam tables, the enthalpy of vaporization of water at 25 °C is hfg ð25 ° CÞ = 2442:3 kJ=kg Convert this to enthalpy of vaporization on a molar basis as shown.

54

1

hfg ð25 ° CÞ = 2442:3

kJ kg

1 kg 1000 g

Mass and Energy Balances

18 g = 43:96 kJ=mol mol

From the combustion table, mol water in the product stream is 1000 mol = 45 mol N_ H2 OðvÞ = ð0:04 kmol þ 0:005 kmolÞ kmol To obtain the actual heating value of 1 kg of coal, subtract the product of moles of water vapor formed and the molar enthalpy of vaporization of water at 25 °C from the standard heat of combustion of 1 kg coal that has just been calculated. AHV = H 0combn - NH2 OðvÞ

H vapn:

kJ 45 mol H2 O kg coal kg coal = - 31404 kJ=kg coal = - 33382

43:96 kJ mol H2 O

C. Use the overall plant efficiency to calculate the rate of heat input required for power generation.

ηplant =

_ net _ W W 1000 kW ) Q_ in = net = = 2381 kW _Qin ηplant 0:42

Use the boiler efficiency to calculate the rate of heat generation required from the fuel. Q_ 2381 kW = 2588 kW Q_ fuel = in = 0:92 ηboiler Calculate the rate of fuel supply required by using the actual heating value of the fuel. m_ fuel =

2588 kW Q_ fuel = 0:0824 kg=s ðNote : kW  kJ=sÞ = kJ AHV 31404 kg

m_ fuel = 0:0824

kg s

3600 s = 296:64 kg=hr hr

Practice Problems

55

Practice Problems Practice Problem 1.1 In a steam power plant, coal is burned at the rate of 50 lbm/min. The percentage of ash in the coal is 9% by mass. The combustion of coal takes place in a fluidized bed reactor and the ash in the effluent stream from the reactor is removed by scrubbing with water at a feed rate of 2.6 gpm. Determine the mass percentage of ash in the water stream leaving the scrubber.

Practice Problem 1.2 2-Mercaptoethanol (C2H6OS) is produced by reacting ethylene oxide (C2H4O) with hydrogen sulfide (H2S) as per the following reaction equation. C2 H4 O þ H2 S → C2 H6 OS The reaction takes place in the presence of Thiodiglycol and Zeolites as catalysts. The molecular weights (lbm/lbmol) are, ethylene oxide = 44, hydrogen sulfide = 34, 2-mercaptoethanol = 78. To produce 1000 lbm/hr of 2-mercaptoethanol, determine the amounts (lbm) of reactants required if 25% excess hydrogen sulfide needs to be used.

Practice Problem 1.3 Chlorobenzene is produced by direct chlorination of benzene. However, a parallel reaction that produces dichlorobenzene hexachloride also occurs. The parallel reactions are shown here. C6 H6 þ Cl2 → C6 H5 Cl þ HCl C6 H5 Cl þ Cl2 → C6 H4 Cl2 þ HCl

rxn 1 rxn 2

The single pass conversion of benzene is 87% and 20% of chlorobenzene produced in the first reaction participates in the second reaction. 100 lbmol/hr benzene and excess chlorine are fed to the reactor. Determine: A. B. C. D.

the theoretical requirement (lbmol/hr) of chlorine. the yield of chlorobenzene. the selectivity of chlorobenzene relative to dichlorobenzene. the extents of the reactions.

56

1

Mass and Energy Balances

Practice Problem 1.4 Methanol can be produced by the reaction of carbon dioxide and hydrogen as per the following reaction: CO2 þ 3H2 → CH3 OH þ H2 O: The fresh feed available is 500 kmol/hr with 2% inerts and the rest being the reactants in stoichiometric proportion. The feed to the reactor consists of fresh feed and the recycle stream. The single pass conversion of carbon dioxide in the reactor is 25%. As a result, the effluent stream from the reactor is a gas mixture of unreacted reactants, products, and inerts. The reactor effluent stream is fed to a condenser where methanol and water condense and the non-condensable carbon dioxide, hydrogen, and inerts are removed to be part of the recycle stream. A portion (10%) of the non-condensable gas stream is purged to prevent the accumulation of inerts in the system and the rest is recycled. It is desired to achieve an overall conversion of 80% of carbon dioxide to methanol. Molecular weights (kg/kmol): CH3 OH = 32, H2 O = 18: Determine: A. B. C. D. E. F.

the production rate of methanol (kg/hr). the rate of condensation of water. the molar flow rate and composition of the recycle stream. the molar flow rate and composition of the reactor feed stream. the molar flow rate and composition of the gas stream from the reactor. the molar flow rate and composition of the purge stream.

Practice Problem 1.5 In an instant coffee plant, 10000 kg/hr of coffee extract containing 20% solids is concentrated using a vacuum evaporator which is designed to produce a concentrated extract with 50% solids suitable for freeze drying. However, it has been determined that a concentrate with 40% solids prior to the freeze-drying process results in better quality. If the operating condition within the evaporator remains the same (the evaporator continues to achieve a concentration change from 20% solids to 50% solids), determine the following parameters after the required change is made to meet the new requirements by a fraction of the feed bypassing the evaporator. A. the rate of production of the concentrated solution B. the rate of evaporation of water C. the fraction of the fresh feed needed to bypass the evaporator to achieve a product concentration of 40% solids.

Practice Problems

57

Practice Problem 1.6 A process requires a supply of 2,500 gal/hr of 12% (by mass) hydrochloric acid solution and it can tolerate a variation of ±0.15% in the concentration of hydrochloric acid. A system with a buffer tank is designed such that an alarm will be activated when the concentration of hydrochloric acid goes above 12.15% or falls below 11.85% in the tank as well as in the supply stream to the process. The contents of the tank are well mixed resulting in a uniform concentration of the acid throughout the tank. The capacity of the buffer tank is 5000 gallon and it contains 4500 gallons of 12% hydrochloric acid solution at an instant when the concentration of the inlet stream to the buffer tank spikes up to 16%. Under normal operating conditions, the inlet stream to the buffer tank is 2500 gal/hr of 12% hydrochloric acid solution. The concentrations given are on a mass basis and the density of all the liquid solutions are 8.92 lbm/gal. Determine the time it takes for the alarm to be activated after the spike in the inlet hydrochloric acid solution.

Practice Problem 1.7 Rework Example 1.14 using the standard heats of combustion data provided here. 0

H c,C2 H6 = - 1561 kJ=mol, 0

H c,C2 H4 = - 1411 kJ=mol, 0

H c,H2 = - 286 kJ=mol

Practice Problem 1.8 The water gas shift reaction (WGSR) is the reaction of carbon monoxide with steam to produce carbon dioxide and hydrogen and it is used in the production of hydrogen for industrial applications. 600 lbmol/hr of carbon monoxide and 30% excess steam are fed to a reactor at 600 °F and the product gases leave the reactor at temperature of 700 °F. The product gas leaving the reactor is cooled to 400 °F in a heat exchanger. The following thermodynamic data is available. Standard heats of formation (Btu/lbmol): 0

H f ,CO2 = - 169, 175,

0

H f ,CO = - 47, 506,

0

H f ,H2 OðgÞ = - 103, 968

Molar enthalpies of gases relative to 77 °F (25 °C) in Btu/lbmol:

58

1

Mass and Energy Balances

H CO ð600 ° FÞ = 3720

H H2 OðgÞ ð600 ° FÞ = 4359

H H2 OðgÞ ð700 ° FÞ = 5233

H CO2 ð700 ° FÞ = 6429

H H2 ð700 ° FÞ = 4319

H CO2 ð400 ° FÞ = 3129

H H2 ð400 ° FÞ = 2231

H H2 OðgÞ ð400 ° FÞ = 2652

Determine: A. the standard heat of reaction for the water gas shift reaction (WSGR). B. the molar flow rate of the product gas from the reactor in standard cubic feet per minute (SCFM). C. the heat transfer to (or from) the reactor in Btu/hr, by using the standard heat of reaction determined earlier. D. the heat duty of the heat exchanger in Btu/hr.

Practice Problem 1.9 A liquid hydrocarbon fuel has a mass composition of 82% carbon and 18% hydrogen. The specific gravity of the fuel is 0.72. Calculate the kmol of air required for the complete combustion of 1 liter of the liquid fuel.

Practice Problem 1.10 Determine the dew point (in °C) of the product gas in Example 1.19 if the product gas discharges at standard atmospheric pressure.

Solutions to Practice Problems Practice Problem 1.1 Solution Draw the schematic diagram of the scrubber as shown.

Solutions to Practice Problems

59

The ash in the effluent stream is completely absorbed by water. Steady-state mass balance for ash results in the following equation. Ash removed from the effluent = ash absorbed by water m_ ash = ðm_ Þðxash Þ = 50

lbm ð0:09Þ = 4:5 lbm= min min

Convert the volume flow rate of water to mass flow rate by using the density of water, 8.34 lbm/gal. m_ w = V_ w ρw = 2:6

gal min

8:34

lbm = 21:68 lbm= min gal

Calculate the mass fraction of ash in water and ash mixture by using the definition of mass fraction of a species. ðxash Þwater =

4:5 lbm m_ ash min = = 0:1719 ð17:2%Þ lbm m_ ash þ m_ water 4:5 lbm þ 21:68 min min

60

1

Mass and Energy Balances

Practice Problem 1.2 Solution Divide the required mass production rate of 2-mercaptoethanol by its molecular weight to obtain the molar production rate of 2-mercaptoethanol. 1000 lbm m_ C2 H6 OS hr N_ C2 H6 OS = = = 12:82 lbmol=hr lbm M C2 H6 OS 78 lbmol From the stoichiometric reaction equation, Molar flow rate ethylene oxide required = molar rate of 2-mercaptoethanol produced = 12.82 lbmol/hr. Multiply the molar flow rate ethylene oxide by its molecular weight to obtain the mass flow rate required. lbmol m_ C2 H4 O = N_ C2 H4 O ÞðM C2 H4 O Þ = 12:82 hr

44

lbm = 564:1 lbm=hr lbmol

Also, from the stoichiometric equation, Stoichiometric molar flow rate of hydrogen sulfide required = molar rate of 2-mercaptoethanol produced = 12.82 lbmol/hr. Multiply the molar flow rate hydrogen sulfide by its molecular weight to obtain the stoichiometric mass flow rate required. lbmol m_ H2 S,stoich: = N_ H2 S,stoich: ðM H2 S Þ = 12:82 hr = 435:9 lb=hr

34

lbm lbmol

Since 25% excess hydrogen sulfide is to be used, multiply the preceding result by 1.25 to get the actual mass flow rate of hydrogen sulfide required. m_ H2 S,actual = ð1:25Þðm_ H2 S,stoich: Þ = ð1:25Þ 435:9 = 544:9 lbm=hr

lbm hr

Solutions to Practice Problems

61

Practice Problem 1.3 Solution As per the stoichiometric equation for rxn 1, one mole of benzene reacts with one mole of chlorine. Assume equimolar feed of chlorine and benzene to the reactor. Basis 100 lbmol/hr of benzene and excess chlorine fed to the reactor. Due to 87% conversion of benzene, the product stream will consist of unreacted benzene, unreacted chlorine, chlorobenzene, hydrogen chloride, and dichlorobenzene. Draw the schematic diagram for the chlorination reactor as shown.

A. From the stoichiometric equation for rxn 1, Mols chlorobenzene produced in rxn 1 = mols benzene reacting. Therefore, N_ P,C6 H5 Clðrxn1Þ = 0:87N_ F,C6 H6 = ð0:87Þ 100

lbmol hr

= 87 lbmol=hr Also, from the stoichiometric equation for rxn 1, Mols chlorine required in rxn 1

= mols benzene reacting in rxn 1 = 87 lbmol/hr. Twenty percent of chlorobenzene produced in rxn 1 will react in rxn 2. From the stoichiometry of rxn 2, Moles chlorine required in rxn 2 = moles chlorobenzene reacting in rxn 2

62

1

Mass and Energy Balances

lbmol hr = 17:40 lbmol=hr = ð0:20Þ 87

Theoretical amount of chlorine required = 87

lbmol lbmol þ 17:4 = 104:4 lbmol=hr hr hr

B. Net amount of the desired chlorobenzene produced, N_ D = chlorobenzene produced in rxn 1 - chlorobenzene consumed in rxn 2 lbmol lbmol - 17:4 hr hr = 69:6 lbmol=hr = 87

Under ideal conditions, that is, no side reactions and complete conversion of benzene, N_ D,ideal = 100 lbmol=hr: Calculate the percent yield of chlorobenzene using Eq. 1.9b. Percent yield of chlorobenzene = 100

N_ D

N_ D,ideal

= 100

69:6 lbmol hr = 69:6% 100 lbmol hr

C. Using the results from the preceding section, Moles of the desired chlorobenzene produced = N_ D = 69:6 lbmol=hr Dichlorobenzene is the undesired product. From the stoichiometry of rxn 2, N_ U = moles dichlorobenzene produced = moles chlorobenzene reacting in reaction(2) = 17.4 lbmol/hr. Calculate the selectivity of chlorobenzene relative to dichlorobenzene using Eq. 1.8. _ 69:6lbmol hr Selectivity of chlorobenzene over dichlorobenzene = NN_ D = 17:4lbmol = 4:0 U

hr

D. Use benzene as the reference species. Benzene participates only in rxn 1. Apply Eq. 1.7c to benzene.

Solutions to Practice Problems

63

N_ C6 H6 = N_ C6 H6 ,F - ξ_ 1 ) lbmol lbmol _ 13 = 100 - ξ1 ) hr hr ξ_ 1 = 87 lbmol=hr Use chlorobenzene as the reference species. There is no chlorobenzene in the feed. Chlorobenzene is produced in rxn 1 and consumed in rxn 2. Apply Eq. 1.7c to chlorobenzene. N_ C6 H5 Cl = N_ C6 H5 Cl,F þ ξ_ 1 - ξ_ 2 ) lbmol mol _ 69:6 =0 þ ξ1 - ξ_ 2 ) hr s lbmol lbmol lbmol = 87 - 69:6 = 17:4 lbmol=hr ξ_ 2 = ξ_ 1 - 69:6 hr hr hr

Practice Problem 1.4 Solution Draw the schematic diagram for the process with the following nomenclature and known quantities: Fresh feed, F = 500 kmol/hr Inerts, N_ I = 0:02F = 0:02 × 500 kmol hr = 10 kmol=hr The moles of reacting species, CO2 and H2 in the feed stream are in stoichiometric proportion. Hence, kmol kmol - 10 = 490 kmol=hr N_ react:species = N_ F - N_ I = 500 hr hr

kmol react:species N_ CO2 ,F = 490 hr = 122:5 kmol CO2 =hr

kmol react:sp N_ H2 ,F = 490 hr = 367:5 kmol H2 =hr

1 kmol CO2 4 kmol react:species

3 kmol H2 4 kmol react:sp

G: non-condensable gas stream from condenser (CO2, H2), of which R: recycle stream, R = 0.90G

64

1

Mass and Energy Balances

P: purge stream, P = 0.10G L: liquid product stream consisting of CH3OH and H2O Single pass conversion of CO2, X single,CO2 = 0:25 Overall conversion of CO2, X overall,CO2 = 0:80

A. The overall conversion of carbon dioxide to methanol is 80% and from the reaction stoichiometry, one mole of carbon dioxide produces one mole of methanol. Calculate the molar rate of production of methanol using molar balance and overall conversion as shown. 0:8 kmol CO2 reacting N_ CH3 OH,prod: = ð122:5 kmol CO2 fed=hrÞ 1 kmol CO2 fed 1 kmol CH3 OH produced 1 kmol CO2 reacting = 98 kmol CH3 OH produced=hr Multiply the molar production rate of methanol by its molecular weight to obtain the mass production rate. m_ CH3 OH,prod: = N_ CH3 OH,prod: ðM CH3 OH Þ kmol CH3 OH produced hr = 3136 kg=hr = 98

32 kg kmol

B. From the stoichiometric reaction equation, one mole of water is produced per mole of methanol produced.

Solutions to Practice Problems

65

98 kmol CH3 OH produced N_ H2 O,prod: = hr 1 kmol H2 O produced 1 kmol CH3 OH produced = 98 kmol H2 O produced=hr ×

Multiply the molar production rate of water by its molecular weight to obtain the rate of condensation of water. m_ H2 O,cond: = N_ H2 O,prod: ðM H2 O Þ kmol H2 O produced hr = 1764 kg=hr = 98

18 kg kmol

C. Isolate the reactor and draw the schematic as shown. R1 is the feed stream to the reactor composed of the recycle stream and the fresh feed stream. R2 is the exit stream from the reactor

The single pass conversion of carbon dioxide in the reactor is 25%, that is 25 mole % of carbon dioxide fed to the reactor is converted to methanol, which is eventually condensed to a liquid product, that is, the entire methanol produced in the system is in the exit stream from the reactor. Therefore, 0:25 N_ CO2 ,R1 = N_ CH3 OH,R2 = N_ CH3 OH ðLÞ,prod = 98 kmol=hr ) kmol 98 hr = 392 kmol=hr N_ CO2 ,R1 = 0:25 Calculate the molar flow rate of carbon dioxide in the recycle stream using a mole balance for carbon dioxide at junction 1. moles carbon dioxide into junction 1 = moles carbon dioxide out of junction 1

66

1

Mass and Energy Balances

N_ CO2 ,F þ N_ CO2 ,R = N_ CO2 ,R1 ) N_ CO2 ,R = N_ CO2 ,R1 - N_ CO2 ,F kmol kmol = 392 - 122:5 hr hr = 269:5 kmol CO2 =hr Since carbon dioxide and hydrogen are in stoichiometric proportion (1:3) in the recycle stream, calculate the molar flow rate of hydrogen in the recycle stream as shown. kmol CO2 N_ H2 ,R = 269:5 hr

3 kmol H2 = 808:5 kmol H2 =hr 1 kmol CO2

The inerts do not react. Mole balance for inerts around the red dashed line results in the following equations. molar flow rate of inerts out (in the purge stream) = molar flow rate of inerts in N_ inerts,P = 10 kmol=hr The inerts in the purge stream is 10% of the inerts in the gas stream and the inerts in the recycle stream is 90% of the inerts in the gas stream. Therefore, kmol _ P,inerts 10 hr N N_ inerts,G = = = 100 kmol=hr, and 0:10 0:10 kmol N_ inerts,R = ð0:90Þ 100 = 90 kmol=hr hr Calculate the total molar flow rate of the recycle stream. kmol H2 kmol CO2 N_ R = N_ CO2 ,R þ N_ H2 ,R þ N_ inerts,R = 269:5 þ 808:5 hr hr kmol inerts þ 90 hr = 1168 kmol=hr Calculate the mole fractions of the components in the recycle stream. yCO2 ,R =

kmol CO N_ CO2 ,R 269:5 hr 2 = = 0:23 1168 kmol N_ R hr

Solutions to Practice Problems

yH2 ,R =

yinerts,R =

67 kmol H N_ H2 ,R 808:5 hr 2 = = 0:69 1168 kmol N_ R hr

N_ inerts,R 90 kmolhrinerts = = 0:08 1168 kmol N_ R hr

Therefore, the molar flow rate of the recycle stream is 1168 kmol/hr and the molar composition of the recycle stream is: CO2 = 23%, H2 = 69%, inerts = 8% D. R1 is the feed stream to the reactor. From the solution to part (C), the molar flow rate of carbon dioxide entering the reactor is N_ R1 ,CO2 = 392 kmol=hr Calculate the molar flow rate of hydrogen entering the reactor using a mole balance for hydrogen at junction 1. kmol kmol þ 808:5 N_ H2 ,R1 = N_ H2 ,F þ N_ H2 ,R = 367:5 hr hr = 1176 kmol H2 =hr Calculate the molar flow rate of inerts entering the reactor using a mole balance for inerts at junction 1. kmol kmol N_ inerts,R1 = N_ inerts,F þ N_ inerts,R = 10 þ 90 hr hr = 100 kmol inerts=hr Calculate the total molar flow rate of the reactor feed stream. kmol H2 kmol CO2 N_ R1 = N_ CO2 ,R1 þ N_ H2 ,R1 þ N_ inerts,R1 = 392 þ 1176 hr hr kmol inerts þ 100 hr = 1668 kmol=hr Calculate the mole fractions of the components in the reactor feed stream. yCO2 ,R1 =

392 kmolhrCO2 N_ CO2 ,R1 = = 0:24 1668 kmol N_ R1 hr

68

1

yH2 ,R1 =

yinerts,R1 =

Mass and Energy Balances

1176 kmolhr H2 N_ H2 ,R1 = = 0:70 1668 kmol N_ R1 hr 100 kmolhrinerts N_ inerts,R1 = = 0:06 1668 kmol N_ R1 hr

Therefore, the molar flow rate of the reactor feed stream is 1668 kmol/hr and the molar composition of the reactor feed stream is: CO2 = 24%, H2 = 70%, inerts = 6% E. The gas stream from the condenser, as well as the reactor exit stream consists of unreacted carbon dioxide and hydrogen and the inerts in identical quantities. Since the single pass conversion of carbon dioxide is 25%, the unreacted carbon dioxide is 75% of carbon dioxide in the reactor feed stream. kmol CO2 N_ CO2 ,G = N_ CO2 ,R2 = ð0:75ÞðN_ CO2 ,R1 Þ = ð0:75Þ 392 hr = 294 kmol CO2 =hr From the stoichiometric equation, one mole of carbon dioxide requires three moles of hydrogen on the reactant side. Hence, the molar rate of hydrogen consumption in the reactor will be three times the consumption rate of carbon dioxide in the reactor and the remaining hydrogen will be in the reactor exit stream and in the gas stream emerging from the condenser. Hence, N_ H2 ,G = N_ H2 ,R2 = N_ H2 ,R1 - 3ðN_ CO2 ,R1 - N_ CO2 ,R2 Þ kmol CO2 kmol CO2 kmol H2 = 1176 - 3 392 - 294 hr hr hr = 882 kmol H2 =hr Inerts are not consumed in the reaction. Hence, the molar flow rate of inerts will be identical at the reactor entrance, at the reactor exit, and the gas stream from the condenser. N_ inerts,G = N_ inerts,R2 = N_ inerts,R1 = 100 kmol inerts=hr Calculate the total molar flow rate of the gas stream from the condenser.

Solutions to Practice Problems

69

kmol CO2 kmol H2 þ 882 N_ G = N_ CO2 ,G þ N_ H2 ,G þ N_ inerts,G = 294 hr hr kmol inerts þ 100 hr = 1276 kmol=hr Calculate the mole fractions of the components in the gas stream from the condenser. yCO2 ,G =

kmol CO N_ CO2 , G 294 hr 2 = = 0:23 1276 kmol N_ G hr

yH2 ,G =

yinerts,G =

kmol H N_ H2 ,G 882 hr 2 = = 0:69 1276 kmol N_ G hr

N_ inerts,G 100 kmolhrinerts = = 0:08 1276 kmol N_ G hr

Therefore, the molar flow rate of the gas stream from the condenser is 1276 kmol/hr and the molar composition of the gas stream from the condenser is: CO2 = 23%, H2 = 69%, inerts = 8% F. The molar flow rate of the purge stream is 10% of the molar flow rate of the gas stream from the condenser and the molar flow rate of each component of the purge stream will also be 10% of the molar flow rate of the respective components in the gas stream giving the following results. kmol = 127:6 kmol=hr N_ P = 0:10N_ G = ð0:10Þ 1276 hr

kmol CO2 N_ CO2 ,P = 0:10N_ CO2 ,G = ð0:10Þ 294 = 29:4 kmol CO2 =hr hr

kmol H2 N_ H2 ,P = 0:10N_ H2 ,G = ð0:10Þ 882 = 88:2 kmol H2 =hr hr

70

1

Mass and Energy Balances

kmol inerts N_ inerts,P = 0:10N_ inerts,G = ð0:10Þ 100 = 10 kmol inerts=hr hr Calculate the mole fractions of the components in the purge stream. yCO2 ,P =

yH2 ,P =

yinerts,P =

kmol CO N_ CO2 ,P 29:4 hr 2 = = 0:23 127:6 kmol N_ P hr

kmol H N_ H2 ,P 88:2 hr 2 = = 0:69 127:6 kmol N_ P hr

N_ inerts,P 10 kmolhrinerts = = 0:08 127:6 kmol N_ P hr

Practice Problem 1.5 Solution Draw the schematic diagram for the process with the following nomenclature and representations: FF: fresh feed of coffee extract containing 20% solids = 10000 kg/hr B: Bypass stream having the same concentration of solids as the fresh feed stream FE: feed stream to the evaporator having the same concentration of solids as the fresh feed stream PE: product stream from the evaporator after removal of water P: final concentrated product stream from the process WE: water vapor removed from the evaporator

Solutions to Practice Problems

71

A. Calculate the rate of production of the concentrated solution using mass balance for solids with reference to the red-dashed boundary. Subscript “S” represents solids and subscript “W” represents water. Overall solids balance: Solids flow in = solids flow out S,WE

S,F

F x kg soln Pnew = F S,F = 10000 xnewS,P hr

newS,P

kg solids kg soln kg solids 0:40 kg soln 0:20

= 5000 kg soln=hr

B. Calculate the rate of evaporation of water using overall (total) mass balance with reference to the red-dashed boundary. Overall mass balance: Total mass flow in = total mass flow out FF = W E þ P ) W E = F F - P = 10000

kg soln kg soln - 5000 = 5000 kg water=hr hr hr

C. Calculate the fraction of fresh feed that needs to bypass the evaporator as shown here. Solids balance around the evaporator: Solids flow in = solids flow out

S,FE

S,WE

Overall mass balance at junction 1: Mass flow into junction 1 = mass flow out of junction 1

S,PE

72

1

Mass and Energy Balances

FF = FE þ B ) FE = FF - B Combine the preceding equations and substitute the known quantities. ðF F - BÞð0:20Þ = PE ð0:50Þ F = 10000 kg=hr → kg 10000 - B ð0:20Þ = 0:50PE ) hr 2000 - 0:20B = 0:50PE Substitute for PE in the preceding equation by using the results from overall (total) mass balance at junction 2. Mass flow into junction 2 = mass flow out of junction 2 kg hr kg -B ) PE = 5000 hr

PE þ B = P = 5000

2000 - 0:20B = 0:50PE ) 2000 - 0:20B = 0:50 5000

kg -B hr

) B = 1667 kg=hr

Therefore, the fraction of the fresh feed bypassing the evaporator is 1667 kg B hr = = 0:1667 FF 10000 kg hr

Practice Problem 1.6 Solution Take the reference time for calculations as t = 0 at the instant the HCl concentration spikes to 16% in the inlet solution to the buffer tank. Due to the instantaneous spike in the HCl concentration in the inlet stream, the concentration of HCl in the tank as well as in the outlet stream keeps changing with time making this situation suitable for the application of unsteady-state mass balance equation. xHCl is the mass fraction of HCl in the buffer tank as well as in the exit solution from the buffer tank. Draw the schematic diagram for the process.

Solutions to Practice Problems

73

Calculate the mass flow rate HCl in the inlet stream at the instant of the occurrence of the spike in concentration of HCl (to 16%) by using the given density. m_ HCl,in = ðxHCl,in Þðm_ in Þ = ðxHCl,in Þ V_ in × ρ lbm HCl gal lbm soln 2500 × 8:92 lbm soln: hr gal = 3568 lbm HCl=hr

= 0:16

Calculate the mass of HCl in the buffer tank at any instant after the spike in HCl concentration. mHCl = ðmsoln ÞðxHCl Þ = ðV soln × ρÞðxHCl Þ lbm soln ðxHCl Þ gal lbm HCl

= 4500 gal × 8:92 = 40140 xHCl

Calculate the mass flow rate HCl in the outlet stream at any instant after the spike in HCl concentration. The concentration of HCl in the outlet stream is the same as the concentration of HCl in the tank. m_ HCl,out = ðm_ out ÞðxHCl,out Þ = V_ out × ρ ðxHCl Þ gal lbm soln ðxHCl Þ × 8:92 hr gal = 22300 xHCl lbm HCl=hr

= 2500

Apply the unsteady-state mass balance equation (Eq. 1.12) to HCl and substitute the known values. Each term in the following set of equations has the units lbm HCl/hr and the units are not included for the sake of brevity of equations. m_ HCl,in - m_ HCl,out =

dmHCl ) dt

3568 - 22300 xHCl = 40140

dxHCl dt

74

1

Mass and Energy Balances

Simplify the preceding equation, separate the variables, and integrate with the following limits, when t = 0, xHCl = 0.12 when t = t, xHCl = 0.1215 Note that the time taken for the alarm to sound is t hours after the spike, when the HCl concentration reaches 12.15%. 1 - 6:25xHCl = 11:25 t

1 11:25

dxHCl ) dt

0:1215

dxHCl ) 1 - 6:25xHCl

dt = 0

0:12

t 1 ½lnð1 - 6:25xHCl Þ0:1215 = 0:12 ) - 6:25 11:25 11:25 t = ½ln 0:25 - ln 0:2406 = 0:069 hrs  4:14 min: 6:25 Note: While evaluating the integral on the right-hand side of the preceding equation, the limits are switched due to the negative sign that precedes the constant 6.25.

Practice Problem 1.7 Solution Calculate the standard heat of reaction per mol (gmole) of ethane reacting using Eq. 1.20. 0

0

tH r = i

0 = H c,C2 H6

-

= - 1561

0

-

H ci

H ci

react: 0 H c,C2 H4

kJ mol

i

þ

prod:

0 H c,H2

- 1411

kJ kJ = 136 kJ=mol - 286 mol mol

Since the standard heat of reaction is positive, the reaction is endothermic (absorbs heat) requiring the supply of heat. Calculate the rate of heat supply required for ethane flow rate of 36 kmol/hr (as obtained in the solution to Example 1.14).

Solutions to Practice Problems

kmol Q_ in = 36 hr

75

136

kJ 1000 mol × mol kmol

1 hr = 1360 kW 3600 s

Thus, similar results are obtained for the heat of a reaction, whether we use standard heats of formation or standard heats of combustion of the reactants and products.

Practice Problem 1.8 Solution A. Write the balanced reaction equation for the water gas shift reaction. CO þ H2 OðgÞ → CO2 þ H2 Calculate the standard heat of reaction using Eq. 1.19 and the given standard heats of formation data. 0

Hr =

0

i

Hf

0 = H f ,CO2

þ

0

-

i

products 0 H f ,H2 -

i

Hf

0 H f ,CO

þ

i reactants 0 H f ,H2 O

Btu Btu Btu þ 0 - - 47506 - 103974 lbmol lbmol lbmol = - 17695 Btu=lbmol

= - 169175

B. Calculate the lbmol/hr of steam and the molar flow of product species based on 30% excess steam used and the stoichiometric reaction equation. N_ CO,F = 600 lbmol=hr ðgivenÞ

1 lbmol H2 OðgÞ _ ðN CO,F Þ N_ H2 OðgÞ,F = ð1:30Þ 1 lbmol CO 1 lbmol H2 OðgÞ 1 lbmol CO = 780 lbmol=hr = ð1:30Þ

600

lbmol hr

The product stream will consist of excess steam, carbon dioxide, and hydrogen.

76

1

Mass and Energy Balances

lbmol = 180 lbmol=hr N_ H2 OðgÞ,P = ð0:30ÞðN_ CO,F Þ = ð0:30Þ 600 hr

1 lbmol CO2 lbmol 1 lbmol CO2 N_ CO2 ,P = ðN_ CO, reacting Þ = 600 1 lbmol CO hr 1 lbmol CO = 600 lbmol=hr

1 lbmol H2 lbmol 1 lbmol H2 = 600 N_ H2 ,P = ðN_ CO, reacting Þ 1 lbmol CO hr 1 lbmol CO = 600 lbmol=hr Calculate the total molar flow rate of the product gas. N_ P = N_ H2 O,P þ N_ CO2 ,P þ N_ H2 ,P lbmol lbmol lbmol = 180 þ 600 þ 600 hr hr hr = 1380 lbmol=hr Standard cubic feet (SCF) is the volume of the gas measured at standard conditions of 60 °F (520 °R) and 14.7 psia. Calculate the SCFM of the product gas by using the ideal gas law. 1380

 std N_ RT = V_ SCFM = Pstd = 8, 730 SCFM

lbmol 1 hr psiaft3 × ð520∘ RÞ 10:73 hr 60 min lbmol∘ R 14:7 psia

C. Draw the schematic diagram of the water gas shift reactor and the condenser with all known quantities.

Solutions to Practice Problems

77

Use the given enthalpy of each reactant and product species using the elements and compounds at standard state (1 atm. pressure and 77° F) as the reference state and the results from material balance to construct the following table. Reference state: Elements and compounds at standard state (1 atm. pressure and 77 °F) Species CO H2O(g) CO2 H2

N_ Fi lbmol/hr 600 780 NA NA

N_ Pi lbmol/hr NA 180 600 600

H Fi Btu/lbmol 3720 4359 NA NA

N_ i H i i

H Pi Btu/lbmol NA 5233 6429 4319

H Ei Btu/lbmol NA 2652 3129 2231

= 5:6320 × 106 Btu=hr F

N_ i H i i

= 7:391 × 106 Btu=hr P

Calculate the extent of the reaction by using Eq. 1.7a and carbon monoxide as the reference species. lbmol 0 lbmol N_ - N_ CO,F hr - 600 hr = = 600 lbmol=hr ξ_ = CO ± νCO -1

Calculate the heat transfer rate to the reactor using Eq. 1.21 and the standard heat of reaction obtained earlier (solution to part A). _ 0þ Q_ R = ΔH_ = ξH R = 600

N_ Pi H Pi i

lbmol hr

N_ Fi H Fi i

- 17695

Btu Btu þ 7:391 × 106 hr lbmol

Btu hr = - 8, 858, 000 Btu=hr - 5:632 × 106

The negative sign for Q_ indicates that heat is removed from the reactor.

78

1

Mass and Energy Balances

D. From the enthalpy table,

N_ i H i i

= 3:693 × 106 Btu=hr E

Note that, N_ Pi = N_ Ei Energy balance for the heat exchanger results in the following equations. Rate of energy in = Rate of energy out N_ Pi H Pi = Q_ E þ i

N_ Ei H Ei ) i

Q_ E =

N_ Pi H Pi i

N_ Ei H Ei i

ðHeat is removedÞ

Btu Btu = 7:391 × 106 - 3:693 × 106 hr hr = 3:698 × 106 Btu=hr

Practice Problem 1.9 Solution Basis: 1 liter (L) of liquid fuel Calculate the mass of the fuel using the specific gravity data. SGfuel =

ρfuel = 0:72 ) ρwater

ρfuel = 0:72ðρwater Þ = 0:72 1000

kg m3

)

ρfuel = 720 kg=m3

mfuel = ðρfuel ÞðV fuel Þ = 720

1 m3 kg ð1 LÞ = 0:72 kg fuel 3 1000 L m

Use the mass composition of the fuel and molecular weights to determine the moles of carbon and hydrogen in 1 L of the fuel.

Solutions to Practice Problems

79

mcarbon = 0:82 × mfuel = 0:82 × 0:72 kg = 0:5904 kg C

N carbon =

mcarbon 0:5904 kg C = = 0:0492 kmol C kg M carbon 12 kmol

mhyd: = 0:18 × mfuel = 0:18 × 0:72 kg = 0:1296 kg H

N hyd: =

mhyd 0:1296 kg H2 = = 0:0648 kmol H 2 kg M hyd 2 kmol

Calculate the moles of oxygen required for complete combustion of fuel using the stoichiometric reactions. C þ O2 → CO2 ) mol O2 required = mol C = 0:0492 kmol 1 1 H2 þ O2 → H2 O ) mol O2 required = mol H2 2 2 1 = × 0:0648 kmol = 0:0324 kmol 2 Total moles of O2 required = 0.0492 + 0.0324 = 0.0816 kmol Calculate the kmol of air required per liter of fuel as shown. N air =

0:0816 kmol O2 L fuel

4:76 kmol air = 0:3884 kmol air=L fuel kmol O2

Practice Problem 1.10 Solution Calculate the mole fraction of water vapor in the product gas using the data from the solution to Example 1.19 (copied here for reference). The actual combustion equation derived in Example 1.19 is C9:5 H19:04 þ 21:91O2 þ 82:4N2 → 8:6CO2 þ 0:9CO þ 9:52H2 O þ 8:1O2 þ 82:4N2

80

1

yH2 OðvÞ =

Mass and Energy Balances

N H2 OðvÞ 9:52 = = 0:0869 8:6 þ 0:9 þ 9:52 þ 8:1 þ 82:4 N prod

The stack gases form an ideal gas mixture. For an ideal mixture, Mole fraction = volume fraction = pressure fraction The preceding equation is Eq. 2.17 in Chemical Thermodynamics, Chap. 2. Calculate the partial pressure of water vapor in the product gas as shown. yH2 OðvÞ = PH2 OðvÞ =

PH2 OðvÞ ) P P × yH2 OðvÞ = ð101:3 kPaÞð0:0869Þ = 8:80 kPa

The dew point of the product gas is the saturation temperature at the partial pressure of water vapor. From steam tables (or online steam properties data, https:// www.nist.gov/system/files/documents/srd/NISTIR5078-Tab2.pdf). T sat ðat 8:80 kPaÞ = 43:32 ° C, T dew point

References 1. S.A. Ashrafizadeh, Z. Tan, Mass and Energy Balances – Basic Principles for Calculation, Design, and Optimization of Macro/Nano Systems (Springer International Publishing, 2018) 2. Cohen, Shelly: Chemistry LibreTexts, UC Davis (2020). Download from, https://chem.libretexts. org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Mod ules_(Physical_and_Theoretical_Chemistry)/Thermodynamics/Thermodynamic_Cycles/Hesss_ Law 3. Engineering Notes: Flue Gas Analysis by Orsat Apparatus, Download from, https://www. engineeringenotes.com/thermal-engineering/fuels-and-combustion/flue-gas-analysis/flue-gasanalysis-by-orsat-apparatus-combustion-thermodynamics/49480 4. Engineering Tool Box1: Stoichiometric Combustion, USA (2003),. Download from, https:// www.engineeringtoolbox.com/stoichiometric-combustion-d_399.html 5. R.M. Felder, R.W. Rousseau, L.G. Bullard, Elementary Principles of Chemical Processes, 4th edn. (Wiley, 2015) 6. Himmelblau, David M., Riggs, James B.: Basic Principles and Calculations in Chemical Engineering, 8th Pearson Education (2012) 7. NIST Chemistry WebBook, U.S. Department of Commerce (2021). Download from, https:// webbook.nist.gov/chemistry/

Chapter 2

Chemical Thermodynamics

2.1

Introduction

The principles of chemical thermodynamics are extensively used in the analysis and solution of problems in different areas of chemical engineering including energy balances, chemical reaction engineering, and mass transfer [9, 10]. Chemical thermodynamics includes important topics such as equations of state, chemical equilibrium, phase equilibrium, which are essential in determining the feasibility of chemical reactions and processes. It also provides insights into how processes could be improved to provide optimum results. Finally, the laws of thermodynamics form the basis of energy balance, energy transformations, and energy transfer [1, 6].

2.2

Equations of State

The thermodynamic variables that characterize the state of a gas are absolute pressure (P), Volume (V ), and absolute temperature (T ). Equations of state are relationships between P, V, and T and hence they are also known as P, V, T relationships [1, 6, 10]. The most widely used equation of state is the ideal gas law described in the following section.

2.2.1

Ideal Gas Equation

The ideal gas equation gives an accurate relationship between P, V, and T of a gas at moderate pressures and temperatures [1, 6]. The ideal gas law is a combination of two basic gas laws; the Charles’s law, which states that the volume of a gas is directly proportional to the absolute temperature of the gas, and Boyle’s law, which © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 N. S. Nandagopal, Chemical Engineering Principles and Applications, https://doi.org/10.1007/978-3-031-27879-2_2

81

82

2 Chemical Thermodynamics

states that the volume of a gas is inversely proportional to its absolute pressure. The ideal gas is mathematically represented by Eq. 2.1., to obtain the following equation. PV = mRT

ð2:1Þ

In Eq. 2.1, P is the absolute pressure, V is the volume, m is the mass, R is the individual gas constant, and T is the absolute temperature. The value of R is different for different gases, and it depends on the molecular weight, M, of the gas. However, based on the concept of mols and Avagadro number, it is clear that equal mols of substances will have the same number of molecules of the substance. Based on this concept, a Universal Gas Constant, represented by the symbol R can be used in the ideal gas equation based on the mols of a substance rather than the mass of the substance as shown here. Manipulate the ideal gas equation by multiplying and dividing the right hand side of Eq. 2.1 by the molecular weight of the gas. PV =

m ðR × M ÞT M

ð2:2Þ

The mass of the gas divided by its molecular weight is the number of mols of the gas, represented by the symbol N. The product of the individual gas constant and the molecular weight is the universal gas constant, R, which has the same value for all gases and hence the term Universal is used. Substitute the preceding results into Eq. 2.2. PV = NRT

ð2:3Þ

Equation 2.3 is the ideal gas equation in molar form. The values of the universal gas constant in different units are presented here. Universal gas constant, R 1545 ft-lbf/lbmol-°R 10.73 psi-ft3/lbmol-°R 0.7302 atm-ft3/lbmol-°R 1.987 Btu/lbmol-°R 8.314 kJ/kmol.K 0.08314 bar.m3/kmol.K 83.14 bar.cm3/mol.K 0.0821 L.atm/mol.K 1.987 cal/mol.K Note: mol  g mole The individual gas constant can be obtained by dividing the universal gas constant by the molecular weight of the gas.

2.2

Equations of State

83

Table 2.1 Temperature and pressure terminologies associated with ideal gas

Standard cubic feet (SCF) Actual cubic feet (ACF)

Temperature 0 °C 32 °F 20°C 68 °F 60 °F T°R

Absolute pressure 101.3 kPa 14.7 psia 101.3 kPa 14.7 psia 14.7 psia P psia

Standard cubic meter (SCM) Actual cubic meter (ACM)

15 °C TK

101.3 kPa P kPa

Term Standard temperature and pressure (STP) – IUPAC definition Normal temperature and pressure (NTP)

R=

Molar volume (V) 22.4 m3/kmol 359 ft3/lbmol 24.04 m3/kmol 385.3 ft3/lbmol 379.4 ft3/lbmol 359 TP ft3 =lbmol 23.63 m3/kmol 22:4 TP m3 =kmol

R M

ð2:4Þ

Different terms such as STP, NTP, SCF, ACF, and NCM are used in describing the state of an ideal gas with respect to temperature and pressure [6]. The molar volume of an ideal gas can be calculated using the defined states and Eq. 2.3 and the results are summarized in the following table (Table 2.1). Example 2.1 Calculate the molar volume of carbon dioxide at 150 psia and 150 °F. Solution Convert the temperature to absolute value. T = 460 ° þ 150 ° F = 610 ° R The STP (32 ° F  492 ° R, 14.7 psia) molar volume of any gas is 359 ft3/lbmol. Calculate the molar volume of carbon dioxide at the given conditions by using the principles of volume being directly proportional to absolute temperature and inversely proportional to absolute pressure.

V CO2 , ðat 150 psia, 150 ° FÞ

= 359

ft3 lbmol

14:7 psia 150 psia

610 ° R 492 ° R

= 43:62 ft3 =lbmol Note: The molar volume can also be directly calculated using Eq. 2.3,

V=

V RT = , to obtain the same result: N P

84

2.2.2

2

Chemical Thermodynamics

Real Gas Equations of State

The ideal gas equation (Eqs. 2.1 and 2.3) does not give accurate results at extremely high pressures and extremely low temperatures. This is because the macro volume of the gas is very small at extremely high pressures and at very low temperatures and is comparable to the physical volume occupied by the molecules of the gas by themselves. Gases at very high pressures and very low temperatures are known as Real Gases and the P, V, T relationships associated with real gases are known as Real Gas Equations of State [8–10]. The two most commonly used real gas equations are Van Der Waals equation and Redlich–Kwong equation which are described here.

2.2.2.1

Van Der Waals Equation

P=

RT a - 2 V -b V

ð2:5Þ

In Eq. 2.5, R is the universal gas constant, V is the molar volume of the gas (m3/ kmol or ft3/lbmol), and a and b are known as Van Der Waals constants. The Van Der Waals constants can be calculated using the critical constants (critical pressure, Pc, and critical temperature, Tc) of the gas using the following equations. 2

a=

27R T 2c 64Pc

ð2:6aÞ

RT c Pc

ð2:6bÞ

b=

2.2.2.2

Redlich–Kwong Equation

The Redlich–Kwong equation is also a two-parameter real gas equation similar to Van Der Waals equation. P=

a RT -p 2 V -b TV V þ b

ð2:7Þ

The nomenclature in Eq. 2.7 is similar to the nomenclature used in Eq. 2.6. However, a and b in Eq. 2.7 are Redlich–Kwong constants, different from Van Der Waals constants, and they can be calculated using the following equations.

2.2

Equations of State

85 2

ð2:8aÞ

RT c Pc

ð2:8bÞ

b = 0:0866

2.2.3

5

R T 2c a = 0:4275 Pc

Generalized Compressibility Chart – Compressibility Factor

The generalized compressibility chart is a widely used tool for determining the generalized compressibility factor, Z, used in calculating of the parameters P, V, T for real or non-ideal gases. The ideal gas equation (Eqs. 2.1 and 2.3) is modified for use for real gases by introducing the Generalized Compressibility Factor, which is represented by the symbol Z. The compressibility factor can be included in Eqs. 2.1 and 2.3 resulting in the following equations for real gases. PV = ZmRT

ð2:9Þ

PV = ZNRT

ð2:10Þ

The typical format of generalized compressibility chart [4, 6, 10] is shown in Fig. 2.1. The generalized compressibility factor, Z, can be obtained by using the reduced pressure, Pr, and reduced temperature, Tr, as parameters. The reduced pressure and the reduced temperature can be calculated by the following equations. Pr =

P Pc

ð2:11Þ

Tr =

T Tc

ð2:12Þ

In Eq. 2.11, Pc is the critical pressure and in Eq. 2.12 Tc is the critical pressure, which are available from thermodynamic property tables. Example 2.2 The following data is available for carbon dioxide: Critical pressure, Pc = 74 bar, critical temperature, Tc = 31 ° C. The value of the universal gas constant in relevant units is, R = 0:08314 bar  m3 =kmol  K. Calculate the molar volume of carbon dioxide at a pressure of 30 bar and at a temperature of 80 °C using the following methods and comment on the results. A. B. C. D.

Ideal gas law Van Der Waals equation of state Redlich–Kwong equation of state Generalized compressibility chart

86

2

Chemical Thermodynamics

Fig. 2.1 Typical format of generalized compressibility chart. (Generated by the author, N. S. Nandagopal, P E)

Solution Convert the temperatures to their absolute values T = 273 ° þ 80 ° C = 353 K

T c = 273 ° þ 31 ° C = 304 K

A. Ideal gas equation of state Calculate the molar volume using the ideal gas equation of state (Eq. 2.3). PV = NRT )

bar  m3 ð353 KÞ kmol  K V RT V= = = 30 bar N P 3 = 0:9783 m =kmol 0:08314

2.2

Equations of State

87

B. Van Der Waals equation of state Calculate the Van Der Waals constants using Eqs. 2.6a and 2.6b

a=

3

2 27R T 2c

=

2

barm ð27Þ 0:08314 kmolK ð304 KÞ2

64 × 74 bar = 3:6418 bar  m6 =kmol2

64Pc

3

barm 0:08314 kmolK ð304 KÞ RT c b= = = 0:3415 m3 =kmol 74 bar Pc

Calculate the molar volume using the Van Der Waals equation of state (Eq. 2.5) by trial and error. P=

RT a - 2) V -b V 6 bar  m3 ð353 KÞ 3:6418 kPa  m 2 kmol  K kmol 2 m3 V V - 0:3415 kmol

0:08314 30 bar =

The molar volume obtained from the ideal gas equation can be used for the initial trial and later modified suitably. Consequently, V = 1:24 m3 =kmol ) P = 30:3 bar on the left-hand side of the preceding equation. Therefore, the molar volume is, V = 1:24 m3 =kmol as per the Van Der Waals equation of state. C. Redlich–Kwong equation of state Calculate the Redlich–Kwong constants using Eqs. 2.8a and 2.8b 2

3

2

5

barm 0:08314 kmolK ð304 KÞ2

5

R T 2c a = 0:4275 = ð0:4275Þ Pc

74 bar 1

= 64:34 bar  m6  K2 =kmol 3

barm 0:0866 kmolK ð304 KÞ RT c = = 0:3558 m3 =kmol b = 0:0866 74 bar Pc

Determine the molar volume using the Redlich–Kwong equation (Eq. 2.7) by trial and error.

88

2

P=

Chemical Thermodynamics

RT a -p 2 ) V -b TV V þ b

bar  m3 ð353 KÞ kmol  K 30 bar = m3 V - 0:3558 kmol 1 bar  m6  K2 64:34 kmol 1 2 m3 ð353 KÞ2 V V þ 0:3558 kmol 0:08314

The molar volume obtained from the Van Der Waal’s equation can be used for the initial trial and later modified suitably. Consequently, V = 1:30 m3 =kmol ) P = 29:86 bar on the left-hand side of the preceding equation. Therefore, the molar volume is, V = 1:30 m3 =kmol as per the Redlich–Kwong equation of state. D. Generalized compressibility chart Calculate the reduced pressure and reduced temperature using Eqs. 2.11 and 2.12. Pr =

P 30 bar = = 0:405 Pc 74 bar

Tr =

T 353 K = = 1:16 T c 304 K

Using the reduced pressure and reduced temperature as parameters, determine the generalized compressibility factor as shown in the following graph )Z = 0.89

2.2

Equations of State

89

Calculate the molar volume using Eq. 2.10. PV V

= ZNRT ) V = N ZRT = P bar  m3 ð353 KÞ kmol  K = 30 bar = 0:8707 m3 =kmol ð0:89Þ 0:08314

Comments: The ideal gas law, as well as the real gas equations of state, predicts higher molar volumes compared to the generalized compressibility chart. As a general rule, lower pressures result in lesser number of collisions and consequently less repulsion between molecules. Since the given pressure is considerably lower than the critical pressure of carbon dioxide, the molecules are free to move causing the attraction forces to dominate. The stronger attraction forces result in a compressibility factor less than one (Z < 1) and molar volume less than what is predicted by the ideal gas law.

2.2.4

Ideal Gas Mixtures

An ideal gas mixture will have several different gases as components of the mixture. Each component in the mixture obeys the ideal gas law and the mixture as a whole will also obey the ideal gas law [1, 6]. In an ideal gas mixture, the components are usually represented by subscript i and the variables representing component properties will also have subscript i representing the component. However, the overall mixture properties are represented by variables without subscripts. For example, the variable P represents the pressure of the mixture as a whole and P2 represents the pressure exerted by component “2” in the mixture (also known as partial pressure of component “2.”).

2.2.4.1

Definitions and Laws for Ideal Gas Mixtures

Mass Fraction (xi): The mass fraction of component i is the mass of the component i divided by the total mass of the mixture. xi =

mi m

ð2:13Þ

90

2

Chemical Thermodynamics

Mole Fraction (yi): The mole fraction of component i is the moles of the component i divided by the total moles of the mixture. yi =

Ni N

ð2:14Þ

Partial pressure of Component i (Pi): The partial pressure of component i is defined as the pressure exerted by that component when it occupies the mixture volume, V, at the mixture temperature, T. From the ideal gas law (Eq. 2.1) mi Ri T V

Pi =

ð2:15Þ

Partial volume of Component i (Vi): The partial volume of component i is defined as the volume occupied by that component at the mixture pressure, P, and at the mixture temperature, T. From the ideal gas law (Eq. 2.1) Vi =

mi Ri T P

ð2:16Þ

Since temperature is an intensive property, it is uniform throughout the mixture and therefore the temperature of each component in the ideal gas mixture is the same as the temperature of the mixture itself. From the ideal gas law, PV = NRT (Eq. 2.3), both pressure and volume are directly proportional to the moles of the gas. Hence, for an ideal gas mixture, the mole fraction, the pressure fraction, volume fraction, and mole fraction of each component are identical. For any component, i, in an ideal gas mixture, mole fraction = pressure fraction = volume fraction. yi =

N i Pi V i = = N P V

ð2:17Þ

The two most important laws applicable to ideal gas mixtures are Dalton’s law and Amagat’s law. Dalton’s Law: Dalton’s law states that the sum of partial pressures of all the components in an ideal gas mixture will be equal to the total pressure of the mixture. n

Pi = P i=1

ð2:18Þ

2.2

Equations of State

91

Amagat’s Law: Amagat’s law states that the sum of partial volumes of all the components in an ideal gas mixture will be equal to the total volume of the mixture. n

Vi = V

ð2:19Þ

i=1

Example 2.3 Flue gas from a power plant emerges from a stack at a volume flow rate of 35 m3/ min. The stack exit conditions are 350 °C and 101 kPa. The molar composition of the flue gas and the molecular weights of the components are presented in the following table. Component N2 CO2 H2O (v) SO2 O2

Mole% 71 12 11 4 2

Molecular weight (kg/kmol) 28 44 18 64 32

Determine the mass flow rate of the flue gas in kg/min. Solution At the given temperature and pressure, molar flow rate of the flue gas can be calculated using the ideal gas law (Eq. 2.3). _ ) PV_ = NRT PV_ N_ = RT

ð101 kPaÞ 35 =

m3 min

kJ ðð350 ° C þ 273ÞKÞ kmol  K = 0:6825 kmol= min 8:314

Calculate the average molecular weight of the stack gas mixture using a weighted average based on mole fractions. M=

M i yi i

= M N2 yN2 þ M CO2 yCO2 þ M H2 O yH2 O þ M SO2 ySO2 þ M O2 yO2 = 28 × 0:71 þ 44 × 0:12 þ 18 × 0:11 þ 64 × 0:04 þ 32 × 0:02 = 30:34 kg=kmol

92

2

Chemical Thermodynamics

Multiply the molar flow rate of the stack gas by its average molecular weight to obtain the mass flow rate of the stack gas. m_ = M N_ = 30:34

2.3

kg kmol

0:6825

kmol = 20:71 kg= min min

Thermodynamic Property Tables

Thermodynamic tables and charts provide specific properties (properties per unit mass) of a substance [1, 6, 7]. Specific properties can be multiplied by the mass of the substance in thermodynamic calculations for closed systems and they can be multiplied by mass flow rate of the species for situations involving open systems. The different specific properties are presented here along with their symbols and units. Specific properties are represented by corresponding lower case letters. For example, specific volume is represented by v and the total volume is represented by V. Specific Volume (v): The specific volume of a substance is the volume per unit mass of the substance, and it has units of ft3/lbm (USCS) and m3/kg (SI). Since density is mass per unit volume, specific volume is the reciprocal of density. v=

V 1 = m ρ

ð2:20Þ

Specific Internal Energy (u): The specific internal energy of a substance is the internal energy per unit mass of the substance, and it has units of Btu/lbm (USCS) and kJ/kg (SI). u=

U m

ð2:21Þ

Specific Enthalpy (h): The specific enthalpy of a substance is the enthalpy per unit mass of the substance, and it has units of Btu/lbm (USCS) and kJ/kg (S I). h=

H U þ PV = = u þ Pv m m

ð2:22Þ

Specific Entropy (s): The specific entropy of a substance is the entropy per unit mass of the substance, and it has units of Btu/lbm-°R (USCS) and kJ/kg.K (SI).

2.3

Thermodynamic Property Tables

93

s=

S m

ð2:23Þ

Gibbs Thermodynamic Function (G): The Gibbs thermodynamic function [9, 10] is also known as Gibbs free energy and it is defined as the energy associated with a chemical reaction that can be used to do work (“available energy”). Gibbs function is mathematically defined as follows: G = H - TS

ð2:24Þ

For a reaction occurring at standard conditions and at constant temperature, the change in the standard Gibbs free energy can be used in assessing the spontaneity of the reaction. The mathematical expression for the change in the standard Gibbs free energy at constant temperature can be derived using Eq. 2.24. ΔG0 = ΔH 0 - TΔS0

ð2:25Þ

A reaction occurs spontaneously if ΔG0 < 0

ð2:26Þ

Equation 2.26 is the Gibbs criteria for spontaneity of a chemical reaction. Helmholtz Thermodynamic Function: The Helmholtz function, also known as work function, represents the potential to do work in a closed system at constant volume and temperature [7, 10]. It is the part of the internal energy which can be used for useful work. The “Helmholtz free energy” is mathematically represented as follows: A = U - TS

2.3.1

ð2:27Þ

Steam Tables

One can obtain the specific volume, the specific internal energy, the specific enthalpy, and specific entropy from the steam tables [1, 6]. The steam tables are broadly categorized into Saturated Steam Tables and Superheated Steam Tables. Compressed Liquid Tables are less common in availability and use.

2.3.1.1

Saturated Steam Tables

The saturated steam tables can be used for situations between saturated liquid and saturated vapor, that is, for saturated liquid, for liquid–vapor mixtures, and for saturated vapor. The saturated steam tables can be based either on temperature or

94

2

Chemical Thermodynamics

Fig. 2.2 Saturated steam table excerpt (SI units)

Fig. 2.3 Saturated steam table excerpt (USCS units)

pressure. The state of steam in the saturated region is identified by letter subscripts as described here. Subscript f represents the saturated liquid state. Subscript g represents the saturated vapor state. Subscript fg represents the increase in the value of the property due to transformation from saturated liquid state to saturated vapor state. Excerpts of saturated steam tables are shown here (Figs. 2.2 and 2.3).

2.3.1.2

Superheated Steam Tables

When saturated vapor is further heated, it becomes superheated. Both temperature and pressure have to be specified to get the properties of superheated steam. Therefore, superheated steam has two degrees of freedom (two independent variables). Short excerpts from superheated steam tables in both USCS and SI units are shown here for illustrative purposes (Fig. 2.4).

2.3.2

Phase Diagram for Water

Phase diagrams for water are plotted using two variables involving thermodynamic properties [1, 6]. For example, a pressure–specific enthalpy, P - h, diagram is

2.3

Thermodynamic Property Tables

95

Fig. 2.4 Superheated steam table excerpts (SI and USCS units)

Fig. 2.5 Temperature– specific volume phase diagram for water

plotted using pressure and specific enthalpy as parameters. Typically, phase diagrams consist of phase boundaries, which separate different phases. Figure 2.5 illustrates a temperature–specific volume (T - v) diagram for water. The following points can be observed from the T–v phase diagram for water shown in Fig. 2.5. 1. There are three different sections in the diagram based on the phase boundaries: compressed liquid to the left of the saturated liquid curve, liquid–vapor mixture between the saturated liquid and saturated vapor curves (inside the bell shaped curve), and superheated vapor to the right of the saturated vapor curve.

96

2

Chemical Thermodynamics

2. The path ABCD represents the isobar (constant pressure line) for atmospheric pressure, 14.7 psia. Similar isobars can be drawn for other pressures and the isobar for 50 psia is also shown in Fig. 2.5. 3. The value of the parameters at reference point “A” are P = 14.7 psia and T = 50 ° F. However, the saturation temperature or boiling point at 14.7 psia is 212 °F. Since the temperature at reference point “A” is less than the saturation temperature, the phase at reference point “A” is called as sub-cooled liquid. Also, at 50 °F, the corresponding saturation pressure is 0.18 psia. However, at reference point A, the pressure is P = 14.7 psia, which is greater than the saturation pressure of 0.18 psia. Hence, at reference point “A,” the phase is also known as compressed liquid. Therefore, in the sub-cooled/compressed liquid phase region, T < Tsat at the given pressure and P > Psat at the given temperature. 4. The isobars for 14.7 psia and 50 psia are also shown in the Fig. 2.5 along with their corresponding saturation temperatures (boiling points) of 212 °F and 281 °F, respectively, which are known as saturated liquid points with subscript f used for the variables such as enthalpy. Reference point “B” is characterized by Psat = 14.7 psia and, Tsat = 212 ° F. The locus of all the saturated liquid points will form the saturated liquid curve (phase boundary). 5. Heating water beyond reference point “B” will increasingly produce more vapor until all the liquid has vaporized at reference point “C,” which is known as saturated vapor point with subscript g used for the variables such as enthalpy. The locus of all the saturated vapor points will form the saturated vapor curve. The two saturation curves will meet at the critical point, where there is no distinction between the liquid and vapor phases. 6. Heating the saturated vapor at reference point “C” will result in the formation of superheated vapor. Reference point “D” represents superheated vapor and at this point, P = 14.7 psia and T = 281 ° F > Tsat = 212 ° F. Hence the points in the superheated vapor phase region is characterized by T > Tsat. 7. The path B-C represents an isotherm (constant temperature line) and it consists of a two phase mixture of liquid and vapor, that is, liquid–vapor mixture. Details concerning the liquid–vapor mixture phase are discussed in the next section.

2.3.3

Properties of Liquid–Vapor Mixtures

Liquid–vapor mixtures are characterized by the mass fraction of vapor in the mixture, which is known as the quality of mixture and is represented by the symbol x. The mathematical representation of the quality of a liquid–vapor mixture is shown in Eq. 2.28.

2.3

Thermodynamic Property Tables

Quality, x =

97

mg mass of vapor = mass of L‐V mixture mf þ mg

ð2:28Þ

The quality of a liquid–vapor mixture represents the extent to which vaporization has occurred [6]. The subscript fg is used in representing the change in any specific property due to complete vaporization. For example, vfg, represents the increase in specific volume due to complete vaporization of the liquid. Mathematically, vfg = vg - vf

ð2:29Þ

To find the value of any property of a liquid–vapor mixture, the change in property due to complete vaporization is weighted by the quality of the liquid– vapor mixture and the result is added to the value of the specific property of saturated liquid. The following equations can be used in calculating the properties of liquid– vapor mixtures.

2.3.4

v = vf þ xvfg

ð2:30Þ

u = uf þ xufg

ð2:31Þ

h = hf þ xhfg

ð2:32Þ

s = sf þ xsfg

ð2:33Þ

Properties of Compressed Liquid

The enthalpy of liquids is strongly dependent on temperature and not pressure. Increasing the pressure of a liquid does not increase its enthalpy significantly [1, 6]. Since liquids are essentially incompressible, the density and hence the specific volume of a liquid remains constant despite changes in pressure. The increase in enthalpy of a liquid due to compression can be calculated by using Eq. 2.34. Δhcomprn: = vðP2- P1 Þ

ð2:34Þ

Consider liquid water at ambient conditions of 30 °C and 100 kPa. From saturated steam tables, the saturation pressure at 30 °C is 0.0042 MPa (4.2 kPa). Since P > Psat, water is a compressed liquid at 30 °C and 100 kPa. Also, from the saturated steam tables, the enthalpy of saturated liquid at 30 °C is, hf = 125.73 kJ/kg and the specific volume of saturated liquid at the same temperature is, vf = 0.001 m3/kg, which can be considered constant despite the increase in pressure. Calculate the

98

2 Chemical Thermodynamics

increase in enthalpy due to compression by substituting the known values into Eq. 2.34. Δhcomprn: = vðP2 - P1 Þ = 0:001

m3 ð100 kPa - 4:2 kPaÞ kg

= 0:0958 kJ=kg m3 kN m3 kN  m kJ × Note : kPa ×    kg m2 kg kg kg The increase in enthalpy due to compression is added to the enthalpy of saturated liquid at the given temperature of 30 °C to obtain the enthalpy of the compressed liquid at 30 °C and 100 kPa. hcomp:liq = hf at

30 ° C

þ Δhcomprn = 125:73

kJ kJ þ 0:096 = 125:83 kJ=kg kg kg

The enthalpy of the compressed liquid is almost equal to the enthalpy of the saturated liquid at the given temperature [6]. For compressed liquids we can use the properties of the saturated liquid at the given temperature without any significant loss in accuracy. Example 2.4 Determine the specific enthalpy of steam under the following conditions: A. P = 20 psia, T = 160 ° F B. P = 20 psia, x = 0.60 C. P = 20 psia, T = 360 ° F Solution A. From the steam tables (or from online steam property calculators – Spirax Sarco Steam Tables https://www.spiraxsarco.com/resources-and-design-tools/steamtables/saturated-water-line#article-top), at 20 psia, Tsat = 228 ° F > 160 ° F (given temperature of steam). Therefore, steam is a sub-cooled (compressed) liquid at the given conditions of temperature and pressure and its enthalpy is the same as that of the saturated liquid at the given temperature. hð20 psia, 160 ° FÞ = hf at 160 ° F = 127:96 Btu=lbm

2.3

Thermodynamic Property Tables

99

Note: From the excerpt of the table shown here, at 160 ° F, Psat = 4.74 psia. Since the given pressure, 20 psia is greater than the saturation pressure, steam is a compressed liquid at the given conditions. B. Since the quality of steam has been specified, steam is a liquid–vapor mixture with 60% vapor by mass. Determine the enthalpy of this liquid–vapor mixture using Eq. 2.32 and data from steam tables (excerpt shown here for reference) or online resources as shown.

h = hf þ xhfg

20 psia

= 196:25 þ 0:6 × 959:94 = 772:21 Btu=lbm

100

2

Chemical Thermodynamics

C. The given temperature, T = 360 ° F > T sat = 228 ° Fðat 20 psiaÞ

Therefore, steam is superheated at the conditions specified. Determine the enthalpy of steam from the 20 psia superheated steam table as shown in the table excerpt here. hð20 psia, 360 ° FÞ = 1220:2 Btu=lbm Example 2.5 A well-insulated piston-cylinder arrangement with inner diameter 9 in. contains 0.4 ft3 steam at 20 psia and 300 °F. Steam expands at constant pressure, pushing the piston a distance of 10 in. At 20 psia and 300 °F, the relevant properties of steam are: specific enthalpy, h = 1191.5 Btu/lbm and specific volume, v = 22.38 ft3/ lbm. Calculate the change in internal energy of steam due to the expansion.

2.3

Thermodynamic Property Tables

101

Solution This problem can be solved by applying the first law for closed systems (energy balance for closed systems, Eq. 1.14) [2]. With the piston-cylinder arrangement as the reference, Energy added–energy removed = change in internal energy, ΔU: In this case, there is no energy or heat added to the system since the piston is well insulated. Energy is removed due to the expansion of steam at constant pressure, producing work. Hence, ΔU = –energy removed = –work of expansion = –PΔV There is sufficient information to calculate the increase in volume due to expansion and it is calculated as follows: ΔV = Acs ΔL =

π × ð9 inÞ2 ð10 inÞ = 636:17 in3 4

- W = - PΔV = - 20

lbf in2

636:17 in3 = - 12, 723 in‐lbf

Convert the work to Btu as shown. - W = - 12, 723 in‐lbf ×

1 ft 1 Btu × = - 1:363 Btu 12 in 778 ft‐lbf

ΔU = - W = - 1:363 Btu ðdecrease in internal energyÞ Alternate Solution: Since this is a closed system, the mass of steam is constant. However, the increase in volume is 636.17 in3  0.3687 ft3 as shown before. Calculate the final specific volume as shown. v2 =

ð22:38 þ 0:3687Þ ft3 = 22:75 ft3 =lbm lbm

Interpolate to determine the final enthalpy from superheated steam tables as shown.

102

2

Chemical Thermodynamics

h2 20 psia, 22:75 ft3 =lbm = 1196:1 Btu=lbm Calculate the mass of steam using the initial volume and initial specific volume. m=

0:40 ft3 V1 = = 0:0179 lbm ft3 v1 22:38 lbm

From the defining equation of enthalpy, ΔU = mðΔhÞ - PΔV

Btu Btu - 1191:5 lbm lbm 1 Btu lbf 144 in2 0:3687 ft3 - 20 2 × 2 778 ft‐lbf ft in = - 1:2825 Btu = ð0:0179 lbmÞ 1196:1

There is about a 6% discrepancy between the results from the two methods and this is due to round-off of the values from the steam tables. Example 2.6 Determine the state and enthalpy of steam at 300 kPa and 90 °C.

2.4

Thermodynamic Charts

103

Solution

From the saturated steam table excerpt shown here, at 300 kPa (0.30 Mpa), Tsat = 133.5 °C. The temperature of steam T = 90 °C < Tsat = 133.2 °C. Hence, steam is a sub-cooled (compressed) liquid at the given conditions. For a compressed liquid, the enthalpy is the same as that of saturated liquid at the given temperature. From the table excerpt shown, the enthalpy of the saturated liquid at 90 °C is h = hf at 90 ° C = 376:8 kJ=kg:

2.4

Thermodynamic Charts

Thermodynamic charts facilitate a quick and efficient method of determining thermodynamic properties, especially for isentropic thermodynamic processes, where the entropy remains constant. Essentially, thermodynamic charts are phase diagrams which include thermodynamic properties such as enthalpy and entropy with temperature and pressure as parametric variables [1, 6, 9]. The most commonly used thermodynamic charts are: enthalpy–entropy diagram for steam (also known as Mollier diagram), and pressure–enthalpy diagrams for refrigerants. The features and uses of various thermodynamic charts are discussed in the following sections.

2.4.1

Mollier Diagram

The Mollier diagram is a phase diagram for water plotted with specific enthalpy on the y-axis and specific entropy on the x-axis as shown in Fig. 2.6. The additional

104

2

Chemical Thermodynamics

Fig. 2.6 Mollier diagram for steam (SI units). (Generated by the author, N.S. Nandagopal, P E)

parameters used in the Mollier diagram are constant pressure lines, constant temperature curves, and constant quality, (x) curves in the saturated region [1, 6]. Note: The Mollier Diagram for steam in USCS units presented in Fig. 2.7, uses ‘constant moisture percent’ instead of quality. For example, constant moisture of 5% implies that the liquid -vapor mixture of steam has 5% liquid (or condensate) and 95% vapor, which is the same as quality, x = 0.95. Example 2.7 and Practice Problems 2.3, 2.4, and 2.5 illustrate the use of the Mollier diagram. The Mollier Diagram in SI units is shown in Fig. 2.6 and the Mollier diagram in USCS units is shown in Fig. 2.7. Examples and practice problems illustrate the use of steam tables and Mollier diagrams for energy balances across steam turbines.

2.4

Thermodynamic Charts

105

Fig. 2.7 Mollier diagram USCS units. (Generated by the author, N. S. Nandagopal, P E)

Example 2.7 0.65 kg/s of steam at 0.5 Mpa and 400 °C expands through a turbine producing 400 kW of power. Determine the enthalpy and state of steam exiting the turbine. Solution

106

2 Chemical Thermodynamics

Draw the schematic diagram for the turbine as shown. Determine the enthalpy of steam entering the turbine from the Mollier diagram as shown (Note: this could also be obtained from superheated steam tables or online resources). h1 ð0:5 MPa = 500 kPa, 400 ° CÞ = 3280 kJ=kg

Assuming isentropic expansion (no heat loss, Q_ = 0) across the turbine, determine the exit enthalpy of steam from energy balance around the turbine. Rate of energy in = rate of energy out _t) _ 2þW _ 1 = mh mh h2 = h1 -

_t kJ 400 kW  kJs W = 2665 kJ=kg = 3280 m_ kg 0:65 kgs

From the Mollier diagram, at the intersection of h2 = 2665 kJ/kg and s2 = s1 vertical line (state 2), steam is still superheated and P2 = 40 kPa, T2 = 95 ° C (this constant temperature line is not completely visible in the excerpt of Mollier diagram presented in the solution. The saturated vapor line represents 100 C constant temperature)

2.4

Thermodynamic Charts

2.4.2

107

Pressure–Enthalpy (P–h) Phase Diagram

P–h phase diagrams are extensively used in analysis and calculations involving refrigeration and air-conditioning (HVAC) systems. The P–h phase diagram has a dome shaped phase boundary with saturated liquid on the left phase boundary and saturated vapor on the right phase boundary. The state of the refrigerant is a liquid– vapor mixture between these boundaries inside the dome. To the left of the saturated liquid, state of the refrigerant is compressed liquid and the refrigerant is superheated to the right of the saturated vapor line [1, 6]. The P–h phase diagram for refrigerant R-134a is shown in Fig. 2.8 and the P–h phase diagram for carbon dioxide [10] is shown in Fig. 2.9. The P–h phase diagram for carbon dioxide also includes the solid phase and is useful in the process of making dry ice (solid phase carbon dioxide).

Fig. 2.8 Pressure–Enthalpy (P–h) diagram for refrigerant R-134a. (Generated by the author, N. S. Nandagopal, P E)

108

2

Chemical Thermodynamics

Fig. 2.9 Pressure–Enthalpy (P–h) diagram for carbon dioxide. (Generated by the author, N. S. Nandagopal, P E)

Example 2.8 The pressure of saturated R-134a liquid is decreased from 200 psia to 40 psia using a throttling valve. Determine: A. the initial and final enthalpy. B. the change in temperature. C. the change in entropy. Solution

Only the pressure is reduced across the throttling valve. There is no heat transfer or work involved as shown in the schematic diagram. Energy balance per unit mass of the fluid results in the following set of equations.

2.4

Thermodynamic Charts

109

Energy in = energy out ) h1 = h2 : From the R-134a P–h diagram determine the enthalpy of saturated liquid at 200 psia (state point 1) as shown.

Determine the enthalpies, entropies, and temperatures at state points 1 and 2 using the enthalpy, entropy, and temperature lines as shown.

110

2

Chemical Thermodynamics

The enthalpy lines are vertical and therefore enthalpy remains constant during the throttling process. From the diagram, A. h1 = h2 = 55 Btu/lbm B. At state point 1, T1 = 125 ° F and at state point 1, T2 = 0 ° F. Therefore, the change in temperature is ΔT = T 2 - T 1 = 0 ° F - 125 ° F = - 125 ° F C. At state point 1, s1 = 0:105 Btu=lbm‐ ° R and at state point 2, s2 = 0:12 Btu=lbm‐ ° R: Therefore, the change in entropy is Δs = s2 - s1 = 0:12

Btu Btu - 0:105 = 0:015 Btu=lbm‐ ° R lbm‐ ° R lbm‐ ° R

Note: During a throttling process, pressure decreases at constant enthalpy resulting in temperature decrease and increase in entropy and the fluid state changes from saturated liquid to liquid–vapor mixture due to flash vaporization of some of the liquid. The extent of vaporization will depend on the magnitude of the pressure let down. Entropy increase indicates a greater state of disorder due to the presence of vapor molecules. Example 2.9 Carbon dioxide is flowing in a pipeline where the pressure is 500 psia and the temperature is 20 °F. The gas then flows through a pressure-reducing valve and into a vessel where the pressure is maintained at 100 psia. Determine: A. the phase description, enthalpy, and entropy of carbon dioxide in the pipeline. B. the phase description, enthalpy, and entropy of carbon dioxide in the tank.

2.4

Thermodynamic Charts

111

Solution

A. Locate state point 1 of carbon dioxide in the pipeline on the P–h diagram as shown. At state point 1 (500 psia, 20 °F), h1 = 30 Btu=lbm Carbon dioxide is in a state of sub-cooled liquid since the saturation temperature at 500 psia is greater than 20 °F. Also, at state point 1, the entropy of carbon dioxide is, s1 = 0:06 Btu=lbm‐ ° R From state point 1, the pressure of carbon dioxide is reduced from 500 psia to 100 psia using a throttling valve. Enthalpy remains constant during throttling. From state point 1 (500 psia, 20 °F) draw a vertical line (constant enthalpy line) to intersect with the 100 psia horizontal line. This intersection defines state point 2. h2 = 30 Btu=lbm, P2 = 100 psia At state point 2, carbon dioxide is a liquid–vapor mixture with quality, x = 0.26, that is 26% vapor and entropy s2 = 0.08 Btu/lbm ‐ ° R

112

2.5

2

Chemical Thermodynamics

Vapor–Liquid Equilibrium (VLE)

When a species exists in two phases, that is in both liquid and vapor phases, the concentrations/mole fractions of the species in the two phases reaches an equilibrium value depending on the temperature of the liquid mixture [8, 10]. Consider a binary liquid mixture of benzene and toluene with a particular mole fraction of benzene, xB, in the liquid phase. This mixture will be in equilibrium with a mixture of the two components having benzene mole fraction yB, in the vapor phase. In other words, there is a unique value of yB that maps onto a given value of xB. For any component i, the vapor–liquid equilibrium relationship can be written in a functional form as shown here. yi = f ð x i Þ

ð2:35Þ

The nature of the equilibrium functional relationship depends on several factors including the temperature, the components in the mixture, and the total pressure of the mixture in the vapor phase. VLE information plays a crucial role in the design of distillation columns typically used in separation and purification of components in liquid mixtures.

2.5.1

VLE Relationships for Ideal Solutions

An ideal solution in the liquid phase is similar to an ideal gas mixture in the vapor phase. In an ideal solution, similar molecular interactions exist between molecules within a species as well as between molecules from different species [8, 10].

2.5.1.1

Raoult’s Law

Raoult’s law can be used over the entire range of composition (from mole fraction 0 to 1) for ideal solutions. Raoult’s law states that the partial pressure of component i in the vapor phase is equal to the product of the mole fraction of the same component in the liquid phase and the vapor pressure (or saturation pressure) of component i at the mixture temperature. From the section on ideal gas mixtures (Sect. 2.2.4), the partial pressure of component i is P i = yi P

ðEq:2:17Þ

2.5

Vapor–Liquid Equilibrium (VLE)

113

P is the total pressure in the vapor phase and yi is the mole fraction of component i in the vapor phase. Therefore, Raoult’s law can be mathematically expressed as Pi = yi P = xi Psat i

ð2:36Þ

Example 2.10 A binary mixture of n-hexane (C6H14) and n-octane (C8H18) is at a total pressure 45 psia. Generate the VLE curve for this ideal mixture by considering the saturation temperatures for the components at 45 psia and at least three intermediate temperatures. Solution From, Dalton’s law of partial pressures (Eq. 2.18), Total pressure = Sum of partial pressures in the vapor phase Pi = P ) PC6 H14 þ PC8 H18 = 45 psia ) i

PC8 H18 = 45 psia - PC6 H14 Obtain expressions for partial pressures from Raoult’s law (Eq. 2.36) and substitute into the preceding equation. Also, the sum of mole fractions of the two components in a binary mixture is 1.0. ) xC8 H18 = 1 - xC6 H14 sat xC6 H14 Psat C6 H14 þ ð1 - xC6 H14 ÞPC8 H18 = 45 )

xC6 H14 =

45 - Psat C8 H18 - Psat C8 H18

Psat C6 H14

Obtain the following expression for the mole fraction of n-hexane in the vapor phase by using Raoult’s law (Eq. 2.36). yC6 H14 =

xC6 H14 Psat xC6 H14 Psat C6 H14 C6 H14 = P 45 psia

The VLE curve can be generated by computing the x vs. y data (using the preceding equations and vapor pressure data) at different saturation temperatures as shown in the following table and in the accompanying graph.

114

2

Chemical Thermodynamics

The saturation or vapor pressures can be obtained from NIST Chemistry WebBook, National Institute of Standards and Technology, U.S. Department of Commerce, 2021. Thermophysical Properties of Fluid Systems, https://webbook.nist.gov/chemistry/ fluid/ https://webbook.nist.gov/cgi/fluid.cgi?TLow=230&THigh=340&TInc=10& Applet=on&Digits=5&ID=C111659&Action=Load&Type=SatP&TUnit= F&PUnit=psia&DUnit=mol%2Fl&HUnit=kJ%2Fmol&WUnit=m%2Fs& VisUnit=uPa*s&STUnit=N%2Fm&RefState=DEF Example 2.11 A mixture of 50 mole% water, 20 mole% cyclopentane, and 30 mole% cyclohexane exists in the liquid phase at a temperature of 20 °C. The organic components form a separate ideal mixture phase that is immiscible with water. At this temperature, the pure component vapor pressures (in mm Hg) are: sat sat Psat H2 O = 18, PC5 H10 = 338, PC6 H12 = 78:

Determine the total pressure of the vapor phase. Solution Basis: 100 kmol of the mixture ) 50 kmol water (H2O), 20 kmol cyclopentane (C5H10), 30 kmol cyclohexane (C6H12). Since the organics form a separate immiscible phase, determine the liquid mole fractions of each component in the organic phase. xC5 H10 =

N C5 H10 20 kmol = = 0:4 N C5 H10 þ N C6 H12 20 kmol þ 30 kmol

xC6 H12 =

N C6 H12 30 kmol = = 0:6 N C5 H10 þ N C6 H12 20 kmol þ 30 kmol

Each immiscible liquid phase exerts its own pressure in the vapor phase independently and the total pressure is the sum of the partial pressures of each phase.

2.5

Vapor–Liquid Equilibrium (VLE)

115

Determine the partial pressure of each component in the vapor phase using Raoult’s law (Eq. 2.36) for the organic phase. (The vapor mole fractions can be easily determined from the pressure fractions in the vapor phase). PC5 H10 = xC5 H10 Psat C5 H10 = ð0:4Þð338 mm HgÞ = 135:2 mm Hg PC6 H12 = xC6 H12 Psat C6 H12 = ð0:6Þð78 mm HgÞ = 46:8 mm Hg PH2 O = xH2 O Psat H2 O = ð1:0Þð18 mm HgÞ = 18 mm Hg P= = =

2.5.1.2

PC5 H10 þ PC6 H12 þ PH2 O 135:2 mm Hg þ 46:8 mm Hg þ 18 mm Hg 200 mm Hg

Henry’s Law

Henry’s law is applicable for dilute solutions, that is, for low concentrations of the solute in the solvent. Henry’s law is particularly useful for dissolved gases in liquid [8, 10]. Henry’s law can be mathematically expressed in many different forms by using the appropriate Henry’s law constant. Pi = yi P = H i xi

ð2:37Þ

Pi = yi P = H i ci

ð2:38Þ

In Eq. 2.38, ci is the concentration of species in the liquid phase (mol/L or kmol/ m3 or lbmol/ft3). The corresponding units for Henry’s law constants are: Hi = Hi =

yi P  kPa or psia or atm xi

yi P kPa  ci kmol=m3

or

psia lbmol=ft3

or

ð2:37aÞ atm mol=L

ð2:38aÞ

Example 2.12 The concentration of dissolved carbon dioxide in salt water at 20 °C is determined to be 1.5 × 10-5 mol/L. The partial pressure of carbon dioxide in air is determined to be 0.45 mm Hg, where the local atmospheric pressure is (7555555) 750 mm Hg. Determine the Henry’s law constant for the solution of carbon dioxide in salt water in atm/(mol/L).

116

2

Chemical Thermodynamics

Solution Calculate the Henry’s law constant for carbon dioxide using Eq. 2.38. PCO2 0:45 mm Hg = = 3 × 104 mm Hg=ðmol=LÞ cCO2 1:5 × 10 - 5 mol=L = 40 atm=ðmol=LÞ, since 1 atm = 750 mm Hg

H CO2 =

2.5.2

VLE Relationships for Non-ideal Systems

In a non-ideal solution, the molecular interactions between dissimilar molecules are either greater or lesser than the molecular interactions between similar molecules. Consider a liquid solution of acetone and water. The attraction forces between acetone molecules are primarily dipole – dipole attractions, while hydrogen bonding causes the attraction forces between water molecules. The attraction forces between acetone molecule and water molecule are also due to weak hydrogen bonding. Since the intermolecular forces between different molecules in a solution of acetone and water vary both in type and strength, the solution is non-ideal. Non-ideal solutions do not follow Raoult’s law. A correction factor, called activity coefficient (represented by γ i), is applied to the liquid phase in the solution to account for the non-ideal effects [8, 10]. The VLE relationship for non-ideal solutions can be expressed by the following equation. Pi = yi P = γ i xi Psat i

ð2:39Þ

Activity coefficients are determined by experimental measurements and the experimental results are fitted to a suitable activity coefficient model. A widely used activity coefficient model is the Van Laar model, characterized by the following pair of equations. ln γ 1 =

A12

ð2:40aÞ

x1 1 þ AA12 21 x2

ln γ 2 =

A21

ð2:40bÞ

1 þ AA2112 xx21 In the preceding equations, A12 and A21 are known as Van Laar constants.

2.5

Vapor–Liquid Equilibrium (VLE)

117

Example 2.13 A binary solution of acetone (species 1) and water (species 2) is at 20 °C and contains 30 mole% acetone in the liquid phase. The following thermodynamic data is available: sat At 20 °C, vapor pressures are Psat acetone = 24:7 kPa, Pwater = 2:34 kPa

Van Laar constants for the solution pair of acetone and water are determined to be: A12 = 2.1041 and A21 = 1.5555. Determine: A. the activity coefficients, γ 1 and γ 2. B. the partial pressures of acetone and water in the vapor phase. Solution A. Calculate the activity coefficients using the given mole fractions and Van Laar constants by substituting into Eqs. 2.40a and 2.40b. ln γ 1 =

A12

=

1 þ AA1221 xx12 ln γ 2 =

A21 1 þ AA2112 xx21

=

2:1041 × 0:30 1 þ 2:1041 1:5555 × 0:70

1:5555 × 0:70 1 þ 1:5555 2:1041 × 0:30

= 1:6741 ) γ 1 = 5:3340

= 0:9423 ) γ 2 = 2:5659

B. Calculate the partial pressures of acetone and water in the vapor phase using Eq. 2.39 (acetone is component 1 and water is component 2). P1 = γ 1 x1 Psat 1 = 5:3340 × 0:30 × 24:7 kPa = 39:52 kPa P2 = γ 2 x2 Psat 2 = 2:5659 × 0:70 × 2:34 kPa = 4:2029 kPa

2.5.3

Distribution Coefficients for VLE Involving Multicomponent Systems

For multi-component, two-phase liquid–vapor mixtures, the general format of vapor–liquid equilibrium relationship can be written as, yi = K i x i

ð2:41Þ

In Eq. 2.41, Ki is known as the vapor–liquid distribution coefficient and it represents the tendency of a given chemical species to distribute itself preferentially between liquid and vapor phases. yi and xi are the mole fractions of component i in

118

2 Chemical Thermodynamics

the vapor and liquid phases, respectively. The following expressions can be written for Ki depending on the appropriate VLE relationship valid for a given situation. Raoult’s Law: Pi = yi P = xi Psat i ) sat Pi yi = xi ) P Ki =

ð2:42Þ

Psat i P

Henry’s Law: Pi = yi P = H i xi ) Hi yi = x ) P i H Ki = i P

ð2:43Þ

Non-Ideal Solutions: Pi = yi P = γ i xi Pisat ) γ Psat yi = i i x i ) P Ki =

ð2:44Þ

γ i Psat i P

Example 2.14 A mixture of hydrocarbons containing 30 mole% butane, 25 mole% pentane, 20 mole% hexane, 15 mole% heptane, and 10% octane is fed to a flash distillation column at 850 °F and 725 psia. Vapor–liquid distribution coefficients at the given conditions are: Component, i Ki zi (feed mole fraction)

C4H10 1.86 0.30

C5H12 1.33 0.25

C6H12 1.09 0.20

C7H14 0.68 0.15

C8H18 0.21 0.10

Based on a feed of 100 lbmol/hr and a vapor flow rate of 70 lbmol/hr, determine the compositions of the liquid phase and the vapor phase from the column. Solution Draw the schematic diagram for the flash distillation column and perform the required mole balances to obtain equations for liquid and vapor compositions (also use the equilibrium relationship yi = Kixi) as shown (all molar flow rates are in lbmol/hr).

2.5

Vapor–Liquid Equilibrium (VLE)

119

Overall mole balance: F = L þ V ) L = F - V = 100 - 70 = 30 lbmol=hr Mole balance for component i: Fzi Fzi Fzi xi xi

= Lxi þ Vyi ) = Lxi þ V ðK i xi Þ )

= xi ðL þ VK i Þ ) Fzi 100zi = = ) L þ VK i 30 þ 70 K i zi = and yi = K i xi 0:30 þ 0:70K i

Calculate the liquid and vapor compositions (xi and yi) using the preceding equations. The results are summarized in the following table. Component, i Ki zi xi yi

C4H10 1.86 0.30 0.1873 0.3484

C5H12 1.33 0.25 0.2031 0.2701

C6H12 1.09 0.20 0.1881 0.2050

C7H14 0.68 0.15 0.1933 0.1314

C8H18 0.21 0.10 0.2237 0.0470

120

2.5.4

2

Chemical Thermodynamics

Generating VLE Curve Using Relative Volatility Data

The relative volatility between two components in binary mixtures is the ratio of the K value for the less volatile component (component i) to the K value for the more volatile component (component j). Relative volatility (αij) can be calculated using the following equation.

αij =

Ki = Kj

yi xi yj xj

ð2:45Þ

For binary systems, the mole fraction in the vapor phase, yi, can be related to the mole fraction in the liquid phase, xi, in terms of the relative volatility as per the derivation shown here. yi þ yj = 1:0 ) yj = 1 - yi xi þ xj = 1:0 ) xj = 1 - xi

ð2:46Þ

The two preceding equations can be combined to obtain the following results after some algebraic manipulations.

αij =

yi xi 1 - yi 1 - xi

=

yi =

ð y i Þ ð 1 - x i Þ yi - y i x i ) = ð xi Þ ð 1 - yi Þ xi - yi xi

αij xi 1 þ xi αij - 1

ð2:47Þ

After dropping the suffixes i and j in Eq. 2.47, the following expression can be used in generating the VLE curve (y vs. x) by calculating y for various assumed values of x. y=

αx 1 þ ðα - 1Þx

ð2:48Þ

The relative volatility is a measure of the ease of separating the two components in a binary mixture. The relative volatility can be estimated by taking the ratio of the vapor pressures of the two components.

2.5

Vapor–Liquid Equilibrium (VLE)

121

αij =

Psat i Psat j

ð2:49Þ

Example 2.15 Generate the VLE curve for acetone–water mixture using the relative volatility between the two components. Solution Determine the vapor pressures of acetone (component A) and water (component B) at any given temperature using standard data resources for vapor pressures. At 20 °C, the vapor pressures of acetone and water are: sat Psat A = 24:8 kPa and PB = 2:33 kPa

(Source: Thermophysical properties of Fluid Systems, NIST Chemistry Webbook, https://webbook.nist.gov/chemistry/fluid/) Estimate the relative volatility using Eq. 2.49, keeping in mind that i is the more volatile component, which is acetone (component A) in this case. αAB =

Psat 24:8 kPa A = = 10:64 2:33 kPa Psat B

Substitute the preceding value of relative volatility of acetone to water into Eq. 2.48 to obtain an equation for the vapor mole fraction, y, in terms of the liquid mole fraction, x. y=

αx 10:64x = 1 þ ðα - 1Þx 1 þ 9:64x

Use the preceding equation in a spreadsheet to generate y vs. x. The results are tabulated and the corresponding VLE curve is drawn as shown.

122

2.5.5

2

Chemical Thermodynamics

Bubble Point and Dew Point Calculations

Bubble Point: The boiling point of a multicomponent mixture at a given pressure will be different from the boiling points of the pure components at the same pressure. The temperature at which the first bubble of the vapor is generated by heating a multicomponent liquid mixture is called the bubble point of the mixture. Similarly, at a given temperature, the pressure at which the first bubble of vapor appears is known as the bubble pressure. The bubble point of a multicomponent

2.5

Vapor–Liquid Equilibrium (VLE)

123

liquid mixture can be determined by using a trial and error procedure described here [8, 10]. 1. Assume a bubble point temperature. 2. Determine the partial pressure of each component in the vapor phase by using one of the following VLE equations relevant to the given situation.

Pi = γ i xi Psat i

Pi = xi Psat i

Raoult’ s law

Pi = H i xi

Henry’ s law ,

Modified Raoult’ s law for non‐ideal solutions

Pi = P, the total pressure in the vapor phase. If yes, then the assumed

3. Check if i

temperature is the bubble point. If no, assume a different temperature (if Pi > P, assume a lower temperature and if Pi < P, assume a higher i

i

Pi = P.

temperature) and repeat steps 1 to 3 until i

Dew Point: When a multicomponent vapor mixture is cooled at a given pressure, the dew point of the mixture is the temperature at which the first drop of condensate is formed. Similar to bubble point of a multicomponent mixture, the dew point of a multicomponent mixture at a given pressure will be different from the dew points of the pure components at the same pressure. Also, at a specified temperature of a multicomponent vapor mixture, the pressure at which the first drop of condensate is produced is called as the dew pressure. The dew point of a multicomponent liquid mixture can be determined by using a trial and error procedure described here. 1. Assume a dew point temperature. 2. Determine the liquid mole fraction of each component in the liquid phase by using one of the following VLE equations relevant to the given situation.

xi =

xi =

yi P Raoult’ s law Psat i

xi =

yi P Henry’ s law , Hi

yi P Modified Raoult’ s law for non‐ideal solutions γ i Psat i

124

2

Chemical Thermodynamics

xi = 1. If yes, then the assumed temperature is the dew point. If no,

3. Check if i

xi > 1, assume a higher temperature and if

assume a different temperature (if i

xi < 1 assume a lower temperature) and repeat steps 1 to 3 until i

xi = 1. i

Example 2.16 Consider a mixture of hydrocarbons containing 30 mole% n-pentane, 30 mole% n-hexane, 40% n-octane in the liquid phase at a total pressure of 30 bar. The vapor pressures can be calculated using Antoine’s equation: log P = A -

B T þC

P is the saturation pressure in bars and T is the saturation temperature in K and the Antoine’s parameters are listed in the table here. Species C5H12 C6H14 C8H18

A 3.99 3.83 4.65

B 1071 1113 1625

C -40.5 -51.3 -42.6

Determine the bubble point of this mixture at 30 bar. Solution Subscripts “1,” “2,” and “3” represent n-pentane, n-hexane, and n-octane, respectively. Assume bubble point temperatures, and for each assumed temperature, calculate the vapor pressure of each component and then the total pressure in the vapor phase using the following equations based on the given Antoine’s equations for vapor pressures, Raoult’s law for partial pressures, and Dalton’s law for total pressure. Continue the procedure till the total pressure is 30 bar. 1071 T - 40:5 1113 log Psat 2 = 3:83 - T - 51:3 1625 log Psat 3 = 4:65 - T - 42:6

log Psat 1 = 3:99 -

sat sat P = P1 þ P2 þ P3 = x1 Psat 1 þ x2 P2 þ x3 P3 sat sat = 0:3Psat 1 þ 0:3P2 þ 0:4P3

The results are summarized in the following table. Temperatures are in K and pressures are in bar.

2.5

Vapor–Liquid Equilibrium (VLE) Psat 1 ðbarÞ 10.25 45.63 77.26 57.07 51.15 52.30 52.89

T(K) 400 500 550 520 510 512 513

125 Psat 2 ðbarÞ 4.35 22.36 39.65 28.53 25.33 25.95 26.26

Psat 3 ðbarÞ 1.27 12.51 28.02 17.63 14.90 15.42 15.69

P (bar) 4.89 25.40 46.28 32.73 28.90 29.64 30.02

Therefore, the bubble point for this mixture is 513 K (240 °C) Example 2.17 A binary mixture of components 1 and 2 has a mole fraction of 0.40 of component 1 in the vapor phase where the total pressure is 1 bar. The equations for the vapor pressures and activity coefficients for this non-ideal mixture are given here. log Psat 1 = 4:2 -

3022 T þ 201

log Psat 2 = 5:2 -

3559 T þ 221

Psat i ðbar Þ, T ðK Þ ln γ 1 = 0:58x22

and

ln γ 2 = 0:85x21

Determine the dew point temperature and liquid composition of this binary mixture. Solution Since both dew point and liquid composition are unknown quantities, use the following trial procedure to determine them. 1. Using the given total pressure in the vapor phase, calculate the corressponding saturation temperature of each component, at 1 bar, using the appropriate Antoine’s relationship as shown here. log Psat 1 = 4:2 -

3022 3022 ) T sat 1 = 4:2 - log 1 - 201 = 518 K T þ 201

log Psat 2 = 5:2 -

3559 3599 ) T sat 2 = 5:2 - log 1 - 221 = 471 K T þ 221

2. Calculate the initial guess for dew point by using a mole fraction weighted average temperature. sat sat yi T sat i = y1 T 1 þ y2 T 2 = 0:4 × 518 K þ 0:6 × 471 K = 490 K

T= i

126

2

Chemical Thermodynamics

3. With the preceding temperature as the basis, calculate the vapor pressures of the components using the given Antione’s equations. 3022 3022 = 4:2 = - 0:1734 T þ 201 490 þ 201 sat ) P1 = 0:6708 bar log Psat 1 = 4:2 -

3559 3559 = 5:2 = 0:1944 T þ 221 490 þ 221 ) Psat 2 = 1:5646 bar log Psat 2 = 5:2 -

4. Calculate the dew pressure, P, using the following derived equation. Assume the activity coefficients to be 1 for the initial iteration. Py1 Py2 þ =1 ) γ 1 Psat γ 2 Psat 1 2 1 1 = 1:021 bar P= y y2 = 1 0:6 0:4 þ sat þ sat γ 1 P1 γ 2 P2 1 × 0:6708 bar 1 × 1:5646 bar x1 þ x 2 =

5. Using the preceding results, estimate the liquid mole fractions as shown. x1 = x2 =

Py1 1:021 bar × 0:4 sat = 1 × 0:6708 bar = 0:6088 ) γ 1 P1 1 - x1 = 1 - 0:6088 = 0:3912

6. Estimate the activity coefficients using the preceding liquid mole fractions. ln γ 1 = 0:58x22 = ð0:58Þð0:3912Þ2 = 0:0888 ) γ 1 = 1:0929 ln γ 2 = 0:85x21 = ð0:85Þð0:6088Þ2 = 0:3150 ) γ 1 = 1:3703 7. Use the estimates from the preceding steps 1 through 6 and calculate the total pressure in the vapor phase.

2.5

Vapor–Liquid Equilibrium (VLE)

Pcalc

127

= P1 þ P2 sat = x1 γ 1 Psat 1 þ x2 γ 2 P2 = ð0:6088Þð1:0929Þð0:6708 barÞ þ ð0:3912Þð1:3703Þð1:5646 barÞ = 1:2850 bar

Since Pcalc(1.2850 bar) > Pgiven(1.0 bar), iterate steps 2 through 6 by trying a lower temperature and fine tune the results until convergence to Pcalc = 1.0 bar using a spreadsheet for repeating the calculations. The results are summarized in the following table. T (K) 450 490 470 475 473 474 474.1

P1sat (bar) 0.3613 0.6709 0.4969 0.5365 0.5204 0.5284 0.5292

P2sat (bar) 0.7870 1.5645 1.1207 1.2204 1.1797 1.1999 1.2019

Dew P (bar) 0.5349 1.0207 0.7461 0.8083 0.7829 0.7955 0.7968

x1 0.5922 0.6086 0.6006 0.6026 0.6018 0.6022 0.6022

x2 0.4078 0.3914 0.3994 0.3974 0.3982 0.3978 0.3978

y1 1.1013 1.0929 1.0970 1.0959 1.0963 1.0961 1.0961

y2 1.3473 1.3700 1.3588 1.3616 1.3605 1.3610 1.3611

Pcalc (bar) 0.6681 1.2852 0.9356 1.0147 0.9824 0.9984 1.0000

Therefore, the final results are: Dew point temperature =474 K (201 ° C) Liquid composition, x1 = 0.6022 and x2 = 0.3978 Example 2.18 Consider a multicomponent liquid mixture with mole fractions of 0.40 propane (1), 0.40 butane (2), and 0.20 hexane (3) at a temperature of 130 °F. At this temperature, the vapor pressures in psia are: sat sat Psat 1 = 273:4, P2 = 80:61, and P3 = 9:17:

Assuming ideal behavior and validity of Raoult’s law calculate the bubble pressure and the vapor composition at bubble pressure. Solution A. Calculate the partial pressures using Raoult’s law (Eq. 2.36) and add them up to obtain the bubble pressure. P

= P1 þ P2 þ P3 sat sat = x1 Psat 1 þ x2 P2 þ x3 P3

= ð0:40Þð273:4 psiaÞ þ ð0:40Þð80:61 psiaÞ þ ð0:20Þð9:17 psiaÞ = 109:36 psia þ 32:24 psia þ 1:83 psia = 143:43 psia

128

2

Chemical Thermodynamics

Calculate the vapor mole fractions using Eq. 2.17 for ideal gas. y1 =

P1 109:36 psia = = 0:7625 P 143:43 psia

y2 =

32:24 psia P2 = = 0:2248 P 143:43 psia

y3 =

P3 1:83 psia = = 0:0128 P 143:43 psia

Example 2.19 Consider a multicomponent vapor mixture with mole fractions of 0.40 propane (1), 0.40 butane (2), and 0.20 hexane (3) at a temperature of 130 °F. At this temperature, the vapor pressures in psia are: sat sat Psat 1 = 273:4 psia, P2 = 80:61 psia, and P3 = 9:17 psia

Assuming ideal behavior and validity of Raoult’s law calculate the dew pressure and the liquid composition at dew pressure. Solution Calculate the dew pressure, P, using the following derived equation. Use the given vapor composition and the given vapor pressures. x1 þ x 2 þ x 3

Py1 Py2 Py3 þ sat þ sat = 1 ) Psat P2 P3 1 1 P= y y3 y2 1 þ sat þ sat Psat P P 1 2 3 1 = = 35:42 psia 0:4 0:4 0:2 þ þ 273:4 psia 80:61 psia 9:17 psia =

Calculate the liquid composition using Raoult’s law (Eq. 2.36). x1 =

Py1 35:42 psia × 0:4 = = 0:0518 273:4 psia Psat 1

x2 =

Py2 35:42 psia × 0:4 = = 0:1758 80:61 psia Psat 2

x3 =

Py3 35:42 psia × 0:2 = = 0:7725 9:17 psia Psat 3

2.5

Vapor–Liquid Equilibrium (VLE)

2.5.6

129

Azeotropic Mixtures

Azeotropic mixtures are characterized by a state where both the liquid and vapor phases have identical compositions upon boiling of a liquid mixture. After a mixture has reached the azeotropic state, it will not be possible to alter the compositions by distillation. Hence, azeotropic mixtures are also known as constant boiling mixtures. The two common types of azeotropes are positive azeotropes, and negative azeotropes [8, 10]. Positive azeotropes show positive deviations from Raoult’s law, which means that the P-x curve will be higher than what is predicted by Raoult’s law and consequently, the T-x curve will be lower than the ideal curve. As a result, the boiling point of the azeotrope is always less than the boiling points of the pure components of the positive azeotrope. The T-x-y phase diagram of positive azeotropes will exhibit a minimum inflection point. Hence, positive azeotropes are also known as minimum boiling azeotropes. Positive deviations from Raoult’s law occur due to weaker molecular interactions between unlike molecules in solution as compared to stronger molecular interactions between like molecules of pure components. Hence, the molecules in solution can escape into the vapor phase more easily than the molecules in pure components resulting higher vapor pressures (lower boiling points) than what is predicted by Raoult’s law. The higher vapor pressures result in positive deviations from Raoult’s law. The phase diagrams for positive/minimum boiling azeotropes are shown in Fig. 2.10. Examples of minimum boiling azeotropes are: • Ethanol–water (atmospheric pressure, 0.972 mole fraction ethanol, 351 K minimum boiling temperature). • Chloroform–methanol (atmospheric pressure, 0.64 mole fraction chloroform, 327 K minimum boiling temperature) Negative azeotropes show negative deviations from Raoult’s law, which means that the P-x curve will be lower than what is predicted by Raoult’s law and consequently, the T-x curve will be higher than the ideal curve. As a result, the boiling point of the azeotrope is always more than the boiling points of the pure

Fig. 2.10 Phase diagrams for positive/minimum boiling Azeotropes

130

2

Chemical Thermodynamics

Fig. 2.11 Phase diagrams for negative/maximum boiling azeotropes

components of the positive azeotrope. The T-x-y phase diagram of positive azeotropes will exhibit a maximum inflection point. Hence, negative azeotropes are also known as maximum boiling azeotropes. Negative deviations from Raoult’s law occur due to stronger molecular interactions between unlike molecules in solution as compared to weaker molecular interactions between like molecules of pure components. Hence, the molecules in solution need more energy or heat to vaporize as compared to the molecules in pure components resulting in higher boiling points (lower vapor pressures) than what is predicted by Raoult’s law. The lower vapor pressures result in negative deviations from Raoult’s law. The phase diagrams for negative/maximum boiling azeotropes are shown in Fig. 2.11. Examples of maximum boiling azeotropes are:

• Water–formic acid (atmospheric pressure, 0.55 mole fraction formic acid, 382 K maximum boiling temperature). • Nitric acid–water (atmospheric pressure, 0.38 mole fraction nitric acid, 393 K maximum boiling temperature). Example 2.20 Consider a non-ideal binary mixture of components 1 and 2. The equations for the vapor pressures and activity coefficients for this non-ideal mixture are given here. log Psat 1 = 4:2 -

3022 T þ 201

log Psat 2 = 5:2 -

3559 T þ 221

Psat i ðbar Þ, T ðK Þ ln γ 1 = 0:58x22

and

ln γ 2 = 0:85x21

Determine if an azeotrope of the mixture exits at a temperature of 315 K and if so determine the pressure and composition of the azeotrope.

2.5

Vapor–Liquid Equilibrium (VLE)

131

Solution An azeotrope will exist if the continuous function of relative volatility vs. composition ranges from values >1.0 to values > > > vf ) Δvphase

change

= vg - v f ≃ vg =

RT Psat

ð2:61Þ

From the preceding equations, dP Psat Δhphase = dT RT 2

change

ð2:62Þ

Equation 2.62 is the Clausius–Clapeyron equation applicable for phase transition at constant temperature and pressure. Assume Δhphase change to be reasonably constant in the range of interest. Separate the variables in the Clausius–Clapeyron equation and integrate between two states of saturation pressures and corresponding saturation temperatures to obtain the following equation [7, 8, 10]. Psat 2

T2

dP = Δhphase Psat Psat 1

ln

Δhphase Psat 2 = Psat R 1

dT ) RT 2

change T1 change

1 1 T1 T2

ð2:63Þ

Equation 2.63 is very useful in predicting vapor pressures at different state points using a reference state point for which data is available.

136

2

Chemical Thermodynamics

Example 2.21 Given the following vapor pressure data for acetic acid, determine the enthalpy of vaporization of acetic acid at 37 °C. Psat (bar) 0.0236 0.0839 0.2437 0.5126

Temperature, T (K) 300 325 350 370

Solution Convert the temperature of interest to Kelvin. T 2 = 273 ° þ 37 ° C = 310 K Obtain the vapor pressure of acetic acid at 310 K by interpolation. sat Psat 2 = P310 K = 0:0477 bar sat Psat 1 = P300 K = 0:0236 bar

Solve Eq. 2.63. for the enthalpy of vaporization. ln

Psat 2 Psat 1

Δhphase

2.7

change

Δhphase change 1 1 ) T1 T2 R Psat R ln 2sat P1 = 1 1 T1 T2 J 0:0477 bar ln 8:314 mol  K 0:0236 bar = = 54, 409 J=mol 1 1 300 K 310 K

=

Chemical Equilibrium

Chemical reactions can occur in both forward and reverse directions simultaneously and such chemical reactions are known as reversible reactions. When the forward and reverse reactions occur at equal rates, the reaction system reaches a state of equilibrium. The topic of chemical equilibrium involves the study of reaction systems in equilibrium [7–10].

2.7

Chemical Equilibrium

2.7.1

137

Characteristics of Chemical Equilibrium

Consider a closed reactor system, where the reversible gas phase reaction of nitrogen and hydrogen to form ammonia occurs. N2 ðgÞ þ 3H2 ðgÞ Ð 2NH3 ðgÞ Initially, the reaction proceeds in the forward direction due to the presence of pure reactants, nitrogen and hydrogen. As ammonia starts forming, it dissociates into nitrogen and hydrogen due to the reverse reaction. With the passage of time, the forward reaction slows down due to the consumption of the reactants and the reverse reaction speeds up as more and more ammonia is formed. It is important to note that the concentrations of nitrogen and hydrogen would drop to zero without the reverse reaction. However, this does not happen in a reversible reaction system because the reactants are replenished due to the reverse reactions. Also, the product concentration does not keep increasing indefinitely since the product is consumed by the reverse reaction. Eventually, a state of equilibrium or balance is reached when both the forward and reverse reactions occur at the same rates. The concentration vs. time graph (Fig. 2.12) illustrates the situation that has just been described. The following observations can be made about the state of equilibrium. Concentration and reaction profiles: • The equilibrium state achieved is a dynamic equilibrium state. This because both the forward and reverse reactions continue to take place at equal rates even after equilibrium and the reactions do not stop or become static. The reactions are described by the equilibrium arrow (Ð). • At equilibrium, the concentrations of all species of reactants and products remain constant. • At equilibrium, the rates of the forward and reverse reactions are equal, that is, rFWD = rREV. • The same equilibrium state will be achieved even if the left- and right-hand sides of the reaction equations are switched.

2.7.2

Position of Equilibrium – Equilibrium Constant

Consider the following reversible reaction: aA þ bB Ð cC þ dD

138

2

Chemical Thermodynamics

Fig. 2.12 Equilibrium concentration and reaction rate profiles

The equilibrium position attained by the preceding reaction is described by the equilibrium constant generally represented by the symbol KC, where the subscript C denotes concentration-based quantities. The equilibrium constant can be calculated by using the following equation [9, 10]. KC =

½C c ½Dd ½Aa ½Bb

ð2:64Þ

Equation 2.64 is known as the equilibrium constant expression. In Eq. 2.64, the square parentheses indicate concentration of the species, usually molar concentration at equilibrium. a, b, c, d are the stoichiometric coefficients in the balanced chemical reaction equation. The quantities in the equilibrium constant expression are actually based on activity coefficients and hence are dimensionless. Hence, it is a common practice to not require or include units for the equilibrium constant. The right-hand side of Eq. 2.64 can also be evaluated at any point of time before equilibrium is reached and the value at any point of time is represented by the symbol QC, known as the reaction quotient. Thus, at any point of time, QC =

½Cc ½Dd ½Aa ½Bb

ð2:65Þ

and at equilibrium, QC = K C . The value of Qc for a given reaction varies with time as shown in Fig. 2.13. Initially, when pure reactants are mixed, Qc is zero due to the absence of any product species at that point. As the reaction proceeds, the value of Qc increases due to the simultaneous increase in product concentrations and decrease in reactant concentrations.

2.7

Chemical Equilibrium

139

Fig. 2.13 Reaction quotient vs. time

When equilibrium is reached, the value of the reaction quotient attains a constant value equivalent to the equilibrium constant since the concentrations of all species remain constant. The magnitude of the equilibrium constant is indicative of the effectiveness of the reaction system to form the desired products before equilibrium is reached [9, 10]. A large value for Kc indicates the presence of significant amounts of product species in the equilibrium mixture. A small value of Kc indicates that at equilibrium, only a small proportion of the reactants have been converted into products.

2.7.3

Equilibrium Calculations

Equilibrium calculations are usually based on experimental measurements of concentrations of the relevant species at the start of the reaction and at the time when equilibrium is attained. Several types of calculations can be accomplished based on experimental results and the use of the equation for the equilibrium constant (Eq. 2.64). The different types of equilibrium calculations are illustrated in the following sections using solved examples and practice problems.

2.7.3.1

Calculation of the Equilibrium Constant, KC, and Unknown Concentrations

Example 2.22 Dinitrogen tetroxide (N2O4) dissociates into nitrogen dioxide (NO2) as per the following reversible gas phase reaction: N2O4(g) Ð 2NO2(g). 0.50 mol of dinitrogen tetroxide is initially heated in a 1 L flask and when equilibrium is reached the concentrations of dinitrogen tetroxide and nitrogen dioxide are measured to be 0.035 mol/L and 0.285 mol/L respectively. Calculate the equilibrium constant, KC.

140

2

Chemical Thermodynamics

Solution From Eq. 2.64, the equilibrium constant is the ratio of the concentration of the product species raised to the power of the stoichiometric coefficient of each species to the concentration of the reactant species raised to the power of the stoichiometric coefficient of each species. In this case, nitrogen dioxide is the product with a stoichiometric coefficient of 2 and dinitrogen tetroxide is the reactant with a stoichiometric coefficient of 1. Therefore,

KC =

½NO2 2 0:2852 = = 2:321 0:035 ½N2 O4 

Example 2.23 A 3 L flask contains 4.25 g ammonia (NH3), 7 g nitrogen (N2), and 6 g hydrogen at a given point of time during the gas phase reaction: N2 + 3H2 Ð 2NH3, with the equilibrium constant, KC = 0.03 at the given temperature and pressure. Evaluate the direction of the reaction under the given conditions. Solution Calculate the moles of each species at the given state. N N2 =

mN2 7g = g = 0:25 mol M N2 28 mol

N H2 = N NH3 =

mH2 6g = g = 3 mol M H2 2 mol

mNH3 4:25 g = g = 0:25 mol M NH3 17 mol

Calculate the concentration of each species at the given state. N N2 0:25 mol = = 0:0833 mol=L V 3L N 3 mol ½H2  = H2 = = 1:0 mol=L V 3L N NH3 0:25 mol ½NH3  = = = 0:0833 mol=L V 3L ½N2  =

Calculate the reaction quotient using Eq. 2.65.

2.7

Chemical Equilibrium

QC =

141

½NH 3 2 ½Cc ½Dd ð0:0833Þ2 = = = 0:0833 ½N 2 ½H 2 3 ð0:0833Þð1Þ3 ½Aa ½Bb

Since QC > KC, the reaction will proceed in the backward direction resulting in the formation of more reactants, which will lead to higher value of the denominator on the right-hand side of the equation for QC, thus pushing the value of QC towards the equilibrium constant, KC. Example 2.24 Consider the reversible gas phase dissociation reaction of hydrogen iodide gas 2HI Ð H2 þ I2 KC = 0.063 at a given temperature. A 1 L flask is initially filled with 2.3 mols hydrogen iodide. Determine: A. the percent dissociation of hydrogen iodide at equilibrium. B. the equilibrium concentrations of hydrogen and iodine. Solution This problem can be solved by using the “Initial–Change–Equilibrium (ICE)” method. The initial concentration of hydrogen iodide is ½HI =

2:3 mol = 2:3 mol=L L

The flask does not have any hydrogen, H2, or iodine, I2, to begin with. Therefore, ½H2  = ½I2  = 0 mol=L Assume that 2x mols of hydrogen iodide, HI, is consumed before equilibrium is reached. Based on the consumption of 2x mols of hydrogen iodide, x mols of hydrogen and x mols of iodine are produced based on the stoichiometric equation. Using the initial concentrations and consumptions, construct the “ICE” table as shown. Species concentration (mol/L) Initial Change Equilibrium

[HI] 2.3 2x 2.3 - 2x

[H2] 0 x x

[I2] 0 x x

Apply the equation for the equilibrium constant (Eq. 2.64) to the given situation.

142

2

Chemical Thermodynamics

½C c ½Dd ) ½Aa ½Bb ½H ½I  ð xÞ ð xÞ = 2 22 ) 0:063 = ) ½HI ð2:3 - 2xÞ2 x2 ) x = 0:3844 mol=L = ð1:15 - xÞ2 =

KC KC 0:252

Therefore, the equilibrium concentrations are ½HI = 2:3

mol mol = 1:5312 mol=L - 2 0:3844 L L ½H2  = ½I2  = 0:3844 mol=L

2.7.3.2

Equilibrium Constant Based on Partial Pressures KP, and the Relationship Between KP and KC

Consider the reversible gas phase dissociation of ethane (C2H6) into ethylene (C2H4) and hydrogen (H2). C2 H6 Ð C2 H4 þ H2 The equilibrium mixture can be considered as an ideal gas and the concentration of a gas is proportional to its partial pressure since the following relationship can be written by using the ideal gas law: Ci ðmol=LÞ =

Ni P = i ) Pi = C i RT ) Pi α C i V RT

For a reversible gas phase reaction, the partial pressure of each gas species is analogous to the concentration of the species. As a result, the equilibrium constant, KP, can be written in terms of partial pressures. For the reversible gas phase reaction, aAðgÞ þ bBðgÞ Ð cC ðgÞ þ dDðgÞ KP =

PC c PD d PA a PB b

The relationship between KP and KC is

ð2:66Þ

2.7

Chemical Equilibrium

143

K P = K C RT ΔN =

ΔN

where,

N i,prod i

ð2:67Þ

N i,react i

Example 2.25 The reversible gas phase reaction and the composition of the equilibrium mixture at 323 °C are given below. 2SO2 þ O2 Ð 2SO3 1:50 × 10 - 2 M SO3 , 1:25 × 10 - 2 M SO2 , and 1:75 × 10 - 2 M O2 : Determine KC and KP for this reaction. Solution Determine KC using Eq. 2.64.

KC =

½Cc ½Dd ) ½Aa ½Bb

KC =

1:50 × 10 - 2 ½SO3 2 = 39:68 = 2 ½SO2 2 ½O2  1:25 × 10 - 2 1:75 × 10 - 2

2

Calculate KP using Eq. 2.67. In this case, the difference between the sum of moles of products and sum of moles of reactants is N i,prod -

ΔN = i

N i,react = 2 - ð2 þ 1Þ = - 1 i

Calculate the absolute value of the temperature. T = 273 ° + 323 ° C = 596 K KP

= K C RT

ΔN

Latm = ð39:68Þ 0:0821 molK × 596 K = 0:8109

-1

144

2.7.4

2

Chemical Thermodynamics

Disruptions to Equilibrium State – Le Chatelier’s Principle

The same principle involved in Newton’s first law of motion (a body at rest or motion will remain in the state of rest or motion until acted upon by an external force) is also applicable to systems at chemical equilibrium. A system in a state of chemical equilibrium will remain in the same state of equilibrium unless external disturbances are introduced into the system in the form of change in concentration of a species, change in pressure, or change in temperature [9, 10]. The response of the system to external disruptions is described by the Le Chatelier’s principle. Le Chatelier’s principle states that if a system in dynamic chemical equilibrium experiences a change in pressure, temperature, or concentration of species, the equilibrium shifts in the opposite direction to offset the change and re-establish equilibrium. To study the applications of Le Chatelier’s to disruptions in equilibrium, consider a closed reactor system used in the synthesis of ammonia, NH3, from nitrogen N2, and hydrogen H2. This is an exothermic, reversible, gas phase reaction and it can be written as follows. N2 ðgÞ þ 3H2 ðgÞ Ð 2NH3 ðgÞ

ΔH = - 92:4 kJ=mol

It is important to note that this system is already in a state of equilibrium. The effects of disturbances in species concentrations, temperature, and pressure to this system in equilibrium will be examined in the following sections.

2.7.4.1

Responses to Changes in Concentration of a Species

If more reactants are added to the system in equilibrium resulting in the increase in the concentration of the reactants, the system will respond by consuming the reactants thereby reducing the concentration of the reactants, which will also result in the production of more ammonia product.

2.7.4.2

Responses to Changes in Temperature of the System

This is an exothermic reaction in the forward direction resulting in the release of heat into the system. Increasing the temperature of the system is akin to adding heat to the system. As per Le Chatelier’s principle, the system will move in the direction of consuming the heat added to the system, which favors the backward reaction, resulting in the consumption of ammonia, which is the desired product.

2.7

Chemical Equilibrium

2.7.4.3

145

Responses to Changes in Pressure of the System

As per the ideal gas equation, PV = NRT, the pressure of a constant volume gaseous system is proportional to the number of moles in the system at a given temperature. That is, P / N. In the ammonia synthesis reaction, N2 ðgÞ þ 3H2 ðgÞ Ð 2NH3 ðgÞ, there are four moles on the reactant side and two moles on the product side. As an external disturbance, when the system pressure is increased, the system will respond to negate this disturbance and proceed in the direction of lower pressure or lower number of moles, thus favoring the formation of ammonia. Hence, higher pressures are desirable in the synthesis of ammonia. Table 2.2 summarizes the consequences of imposed changes to a system already in a state of chemical equilibrium.

2.7.5

Gibbs Free Energy Change and Chemical Equilibrium

A system at equilibrium has reached its minimum state of energy and there is no “available or free energy” in the system to accomplish useful work. Hence, it follows that there is a relationship between Gibbs free energy changes and the equilibrium constant, KC or KP, which is formulated here [9, 10].

Table 2.2 Disruptions to equilibrium state and system responses Imposed disturbance Addition of reactants Decrease in reactant amount/concentration Decrease in system volume/increase in system pressure Decrease in system pressure/increase in system volume Increase in system temperature by adding heat Decrease in system temperature by removing heat Addition of a catalyst

System response Consumption of reactants Production of reactant by backward reaction Increasing the system volume/decreasing the system pressure Increasing the system pressure/decreasing the system volume Reducing the system temperature by consuming the heat added Increasing the system temperature by heat generation Reaching equilibrium faster

Direction of shift Toward product side Toward reactant side Toward the side with fewer number of moles Toward the side with higher number of moles Toward reactant side in an exothermic reaction and toward product side in an endothermic reaction Toward product side in an exothermic reaction and toward reactant side in an endothermic reaction No change, no shift

146

2

Chemical Thermodynamics

Start with the following thermodynamic identity written in terms of molar quantities. This identity can be derived by taking the differentials of the defining equations of Gibbs free energy, enthalpy, and combining the results with the first law for a closed system as follows: G = H - TS ) dG = dH - TdS - SdT H = U þ PV ) dH = dU þ PdV þ VdP Q - W = ΔU ) TdS - PdV = dU ) dG = - SdT þ VdP Differentiate the preceding identity with respect to P at constant temperature and use the ideal gas relationship for molar volume, V=

RT P

to obtain the following equation, which can be subsequently integrated as shown. ∂G ∂P

= T

RT ) P P2

dP ) P

dG = RT P1

P2 P1

GðT, P2 Þ - GðT, P1 Þ = RT ln

G partial molar Gibbs free energy  μ, the chemical potential. P1 = 1 bar  standard state (denoted by superscript 0). Combine the preceding results to obtain the following relationship for the chemical potential of a species at a given temperature and pressure. μðT, PÞ = μ0 ðT Þ þ RT ln

P P0

μi = μ0i þ RT ln Pi ðsince P0 = 1 barÞ

ð2:68aÞ ð2:68bÞ

Also, since μi =

∂G ∂N i

n

) dG = T,P,N j ≠ i

μi dN i i=1

ð2:69Þ

2.7

Chemical Equilibrium

147

Consider the general reversible gas phase chemical reaction: aA þ bB Ð cC þ dD The change in Gibbs free energy for the preceding reaction can be written as follows by combining the preceding results. ΔG = ΔG0 þ RT ln However,

PcC PdD PaA PbB

PcC PdD PaA PbB

ð2:70Þ

= Q, the reaction quotient for the gas phase reaction at any

given time. At equilibrium, Q = KP, the equilibrium constant based on partial pressures of the species in the reaction mixture at equilibrium. Also, the state of equilibrium is such that there can be no further changes in the energies of the system, and this implies that the change in Gibbs free energy is zero, that is, ΔGeqbm = 0. Substituting the preceding result into Eq. 2.70, an equation can be obtained for the equilibrium constant in terms of ΔG°, the standard Gibbs energy change for the reaction. ΔG0 = - RT ln K

ð2:71Þ

The standard Gibbs energy change for a reaction can be calculated by using any of the following equations. ΔG0 =

G0f

prod:

-

G0f

react:

ð2:72Þ

In Eq. 2.72, G0f = standard free energy of formation of a given species, available from thermodynamic data tables. ΔG0 = ΔH 0 - TΔS0

ð2:18, reproduced here for referenceÞ

ΔH0 and ΔS0 are standard heat of reaction and standard entropy change for the reaction, respectively. Example 2.26 Calculate the equilibrium constant, KP, given the standard Gibbs free energy change for the following gas phase reversible reaction at 50 °C. N2 þ O2 Ð 2NO

ΔG0 = - 67:8 kJ=mol

Solution Convert the temperature to its absolute value, T = 273 ° + 50 ° C = 323 K

148

2

Chemical Thermodynamics

From Eq. 2.71, ΔG0 = - RT ln K P ) KP = e

2.7.6

-

ΔG0 RT

=e

-

- 67:8 kJ mol 0:008314 kJ × 323 K molK

= 9:218 × 1010

Temperature Dependence of Equilibrium Constant – The Van’t-Hoff Equation

The Van’t Hoff equation [5] relates the equilibrium constant to the temperature [10]. d ln K ΔH 0rxn = dT RT 2

ð2:73Þ

In Eq. 2.73, ΔH 0rxn is the standard heat of the reaction. The Van’t Hoff equation can be integrated between two temperature limits to obtain the following definite integral between temperatures T1 and T2. ln

ΔH 0rxn 1 K2 1 = K1 T T R 1 2

ð2:74Þ

In Eq. 2.74, K1 is the equilibrium constant at absolute temperature T1, and K2 is the equilibrium constant at absolute temperature T2. Example 2.27 A gas phase reaction, with a standard heat of reaction of ΔH 0rxn = 139230 Btu/lbmol has a value of KP = 0.0345 at 90 °F. Determine the value of KP for this reaction at 115 °F. Solution Convert the given temperatures to their absolute values. T 1 = 460 ° þ 90 ° F = 550 ° R

T 2 = 460 ° þ 115 ° F = 575 ° R

Substitute the known values into Eq. 2.74 and solve for KP2 at 115 °F. Btu 139, 230 K P2 1 1 lbmol ln = 5:539 = Btu 0:0345 550 ° R 575 ° R 1:987 lbmol‐ ° R ) K P2 = 8:778

Practice Problems

149

Practice Problems Practice Problem 2.1 The critical constants of methane are: Pc = 673 psia and Tc = - 117 ° F. Calculate the pressure exerted by 1.3 lbm of methane in a 0.25 ft3 container at a temperature of 70 °F using A. ideal gas equation. B. generalized compressibility chart.

Practice Problem 2.2 A rigid container has a volume of 70 ft3 and contains 10 lbm of steam at a pressure of 30 psia. Determine the quality of steam.

Practice Problem 2.3 1800 lbm/min of steam at 600 °F and 300 psia expands through a turbine to a final pressure of 10 psia. The isentropic efficiency of the turbine is 90% and the mechanical efficiency of the turbine is 80%. Calculate the horsepower (hp) produced by the turbine.

Practice Problem 2.4 After undergoing the first stage of expansion, steam at 1 MPa is reheated to 500 °C. The reheated steam expands through the second stage turbine to a final pressure of 20 kPa. Determine the enthalpy of the steam leaving the second stage turbine if the isentropic efficiency of the second stage turbine is 92%.

Practice Problem 2.5 Waste heat recovery measures in a process facility results in the availability of superheated steam at 200 psia and 700 °F. The available steam expands in a turbine to a final pressure of 10 psia. If the specific enthalpy of the steam leaving the turbine is 1140 Btu/lbm, determine the isentropic efficiency of the turbine.

150

2

Chemical Thermodynamics

Practice Problem 2.6 The liquid in the tank in Example 2.9 is conveyed via a throttling valve to another tank where the pressure is maintained at 5 psia. Determine the phase description, enthalpy, and entropy of carbon dioxide in the second tank where the pressure is 5 psia.

Practice Problem 2.7 A binary mixture of n-hexane and n-octane is at a temperature of 90 °C and the mole fraction of n-octane in the liquid phase is 0.60. The vapor pressures of n-hexane and sat n-octane at 90 °C are Psat C6 H14 = 189 kPa and PC8 H18 = 33:5 kPa. Calculate the mole fractions of the components in the vapor phase and the total pressure of the gas phase mixture.

Practice Problem 2.8 A binary mixture of propane and butane exists in equilibrium at standard atmospheric pressure of 101 kPa. The mole fraction of propane in the liquid phase is 0.30. The normal boiling points (NBPs) of propane and butane are 231 K and 272 K respectively. Determine the temperature of the liquid mixture by using the boiling point data and standard references for obtaining the vapor pressures.

Practice Problem 2.9 At 30 °C, the Henry’s law constant for a 0.0001 mol/m3 aqueous solution of trichloroethylene (TCE) in equilibrium with atmospheric air is determined to be 1.2 kPa/(mol/m3). Determine the partial pressure of TCE and the mole fraction of TCE in the vapor phase.

Practice Problem 2.10 Chloroform (CHCl3) and methanol (CH3OH) form a non-ideal, binary azeotropic mixture at standard atmospheric pressure and 327 K. The azeotropic composition is 65 mol% chloroform and the Antoine’s equation for calculating the vapor pressure of chloroform is,

Practice Problems

151

log 10 Psat CHCl3 = 4:21 -

1233 , Psat ðbarÞ, T ðKÞ T - 41

Determine the liquid-phase activity coefficient of chloroform in this non-ideal mixture.

Practice Problem 2.11 A binary mixture of water and ethanol is flash distilled at atmospheric pressure. The feed consists of 80 mole% ethanol (component A) and 20 mole% water (component B). The equations for vapor pressures and the average activity coefficients at the given composition and the relevant temperature range are as follows: log Psat A = log Psat B =

1670 T - 40:2 2355 6:21 T þ 7:56 5:37 -

Psat ðbarÞ, T ðKÞ γ A = 1:03,

γ B = 1:99

Determine the temperature and vapor composition at atmospheric pressure in the vapor phase.

Practice Problem 2.12 The relative volatility of cyclohexane with respect to water is 4.33 in the temperature range of interest. If the liquid mole fraction of cyclohexane in a binary solution of water is 0.25, calculate the mole fraction of water in the vapor phase that is in equilibrium with the liquid phase.

Practice Problem 2.13 A mixture of propane, isobutane, and isopentane is at a pressure of 50 psia in the vapor phase with vapor mole fractions of 0.20, 0.30, and 0.50 respectively. The values of the vapor-liquid distribution coefficients, Ki, for each species K can be

152

2

Chemical Thermodynamics

obtained from the nomograph provided. Determine the dew point and liquid composition of this mixture.

Practice Problems

153

Practice Problem 2.14 A mixture of n-hexane, n-heptane, and n-octane with liquid mole fractions 0.30, 0.30, and 0.40 is at its bubble point at a temperature of 80 °F. The values of the vapor-liquid distribution coefficients, Ki, for each species can be obtained from the nomograph provided. Determine the vapor composition and total pressure of this mixture.

154

2

Chemical Thermodynamics

Practice Problem 2.15 The enthalpy of vaporization of water at 20 psia (Tsat = 228 ° F) is 960 Btu/lbm. Analytically determine the vapor pressure of water at 250 °F and compare it with the value from the steam tables.

Practice Problem 2.16 Phosphorus trichloride (PCl3) reacts with chlorine gas (Cl2) to produce phosphorus pentachloride (PCl5) as per the following reversible gas phase reaction: PCl3 ðgÞ þ Cl2 ðgÞ Ð PCl5 ðgÞ The initial and equilibrium concentrations of phosphorus trichloride are 0.30 mol/ L and 0.15 mol/L respectively, while the initial concentration of chlorine is 0.25 mol/ L. Calculate the equilibrium constant, KC.

Practice Problem 2.17 For the reversible gas phase reaction: N2 þ 3H2 Ð 2NH3

K C = 0:0045

At a given temperature, the equilibrium concentrations of ammonia and nitrogen are ½NH3  = 2:15 M ðmol=LÞ and ½N2  = 3:85 M: Determine the equilibrium concentration of hydrogen.

Practice Problem 2.18 For the reversible gas phase reaction: H2 þ CO2 Ð H2 O þ CO, K C = 0:08 at 600 K. The initial concentrations of hydrogen and carbon dioxide are

Solutions to Practice Problems

155

½H2i  = 0:013 M ðmol=LÞ and ½CO2i  = 0:016 M: Determine the concentration of all the species at equilibrium.

Practice Problem 2.19 Consider the reversible gas phase reaction N2 þ 3H2 Ð 2NH3

ΔG0 = - 16:4 kJ=mol at 25 ° C:

At a given non-standard condition, the concentrations of the species are ½N2  = 0:17 mol=L, ½H2  = 0:19 mol=L, and ½NH3  = 0:36 mol=L Determine ΔG at the given condition and the direction the reaction has to proceed to achieve equilibrium.

Solutions to Practice Problems Practice Problem 2.1 Solution Convert the temperatures to their absolute values. T = 460 ° þ 70 ° F = 530 ° R

T c = 460 ° - 117 ° F = 343 ° R

Calculate the moles of methane. N=

m 1:3 lbm = 0:08125 lbmol = lbm M 16 lbmol

A. Ideal gas equation Calculate the pressure using the ideal gas equation of state (Eq. 2.3).

156

2

PV

Chemical Thermodynamics

= NRT ) psia‐ft3 ð530 ° RÞ lbmol‐ ° R 0:25 ft3

ð0:08125 lbmolÞ 10:73

P

NRT = V = 1848 psia =

B. Generalized compressibility chart Assume a value for P, say, P = 2000 psia. Calculate the reduced pressure and temperature using Eqs. 2.11 and 2.12. Pr =

P 2000 psia = = 2:97 Pc 673 psia

Tr =

T 530 ° R = = 1:54 T c 343 ° R

Using the reduced pressure and reduced temperature as parameters, determine the generalized compressibility factor as shown in the following graph )Z = 0.87

Calculate the pressure using Eq. 2.10

Solutions to Practice Problems

157

PV = ZNRT ) P=

ZNRT V ð0:87Þð0:08125 lbmolÞ 10:73

=

psia‐ft3 ð530 ° RÞ lbmol‐ ° R

0:25 ft3

= 1608 psia

Use the preceding calculated pressure and recalculate the reduced pressure. Pr =

P 1608 psia = = 2:39: Pc 673 psia

Use this value of reduced pressure along with the reduced temperature, Tr = 1.54 to obtain a revised value of Z = 0.88. Since the previously obtained value of the compressibility factor was Z = 0.87, this indicates convergence of the Z value to Z = 0.88. Recalculate the pressure using this final value of Z.

P=

ZNRT V

ð0:88Þð0:08125 lbmolÞ 10:73 =

psia‐ft3 ð530 ° RÞ lbmol‐ ° R

0:25 ft3

= 1626 psia Alternate Solution

When the reduced pressure is unknown, as is the case here, the pseudo reduced specific volume, vr, can be used along with the reduced temperature, Tr, to determine the generalized compressibility factor, Z, from compressibility charts where vr curves have been included [1, 10]. The pseudo reduced specific volume is defined by the following equation. vr =

vPc RT c

In the context of this problem, determine the pseudo-reduced molar volume as shown here. First, calculate the molar volume. v=

V 0:25 ft3 = 3:08 ft3 =lbmol = 0:08125 lbmol N

Next, calculate the pseudo-reduced molar volume.

158

2

Chemical Thermodynamics

3

ft 3:08 lbmol ð673 psiaÞ vP vr = c = = 0:56 3 psia‐ft RT c ð 343 ° R Þ 10:73 lbmol‐ °R

Using vr = 0:56 and T r = 1:54 as parameters determine the reduced pressure as shown in the following graph.

From the graph, Pr = 2.35

Solutions to Practice Problems

159

Calculate the actual pressure using Eq. 2.11. P ) Pc P = Pr Pc = 2:35 × 673 psia Pr =

= 1582 psia The discrepancy in the results from the trial and error and the alternate solutions is due to difference in accuracies while reading the two graphs.

Practice Problem 2.2 Solution Calculate the specific volume of steam using Eq. 2.20. v=

V 70 ft3 = = 7:0 ft3 =lbm m 10 lbm

Since the specific volume of steam, v, is between vf and vg, steam is a liquid– vapor mixture. Solve Eqn. 2.30 for the quality, x, and then substitute the known values to obtain the quality.

160

2

Chemical Thermodynamics

v = vf þ xvfg ) ft3 ft3 7:0 - 0:0170 v - vf lbm lbm = 0:5082 ð ≃ 51% vaporÞ = x= vfg ft3 13:74 lbm

Practice Problem 2.3 Solution For isentropic expansion, determine the initial and final enthalpies of steam from the Mollier diagram as shown in the excerpt here. h1 ð300 psia, 600 ° FÞ = 1320 Btu=lbm h2 ð10 psia, s2 = s1 Þ = 1040 Btu=lbm Energy balance (based on unit mass of steam) across the turbine results in the following set of equations [4].

Energy in = energy out ) h1 = h2 þ wt,ideal ) wt,ideal = h1 - h2 = 1315

Btu Btu - 1035 = 280 Btu=lbm lbm lbm

Calculate the horsepower produced by the turbine using the given efficiencies and mass flow rate of steam.

Solutions to Practice Problems

161

1800 _ t,ideal _ t,actual = mw W ηisen ηmech

lbm min

=

280

Btu lbm

0:80 × 0:90

0:0236 hp Btu 1 min

= 16, 520 hp

Practice Problem 2.4 Solution Assuming isentropic expansion, determine the enthalpies of steam entering and leaving the second stage turbine using the Mollier diagram excerpt as shown.

h1 ð1 MPa, 500∘ CÞ = 3480 kJ=kg, and h2s ð20 kPa, s2 = s1 Þ = 2550 kJ=kg Due to the efficiency of the turbine, the actual work produced by the turbine is less than the isentropic or ideal work the turbine can produce. Therefore, the isentropic efficiency of a turbine is the ratio of the actual work to the isentropic work. Isentropic expansion is along the vertical line from state point 1 to state point 2. The actual expansion will be from state point 1 to state point 2′ and therefore the enthalpy of steam leaving the turbine will be h2′. Calculate h2′ using the preceding concepts.

162

2

ηt =

Chemical Thermodynamics

wact: h1 - h2 ′ = ) wid h1 - h2s

h2 ′ = h1 - ηt ðh1 - h2s Þ = 3480

kJ kJ kJ - 0:92 3480 - 2550 kg kg kg

= 2624 kJ=kg

Practice Problem 2.5 Solution

Assuming isentropic expansion, determine the enthalpies of steam entering and leaving the turbine using the Mollier diagram as shown. h1 ð200 psia, 700∘ FÞ = 1370 Btu=lbm h2 ð10 psia, s2 = s1 Þ = 1100 Btu=lbm: The enthalpy of steam leaving the turbine is given and it is h2 ′ = 1140 Btu=lbm Isentropic expansion is along the vertical line from state point 1 to state point 2. The actual expansion will be from state point 1 to state point 2′. The isentropic efficiency of a turbine is the ratio of the actual work to the isentropic work. Calculate the isentropic efficiency of the turbine using the enthalpies (all in Btu/lbm).

Solutions to Practice Problems

ηisen: =

163

wt,actual h1 - h2 ′ 1370 - 1140 = = = 0:85 ð85%Þ 1370 - 1100 wt,ideal h1 - h2

Practice Problem 2.6 Solution

The original tank described in Example 2.9 has saturated liquid carbon dioxide at 100 psia. Consider this as state point 3 as shown in the P–h diagram, with enthalpy h3 = - 10 Btu=lbm At from state point 3 at 100 psia, the pressure is reduced via a throttling valve to 5 psia. Enthalpy remains constant during throttling. From state point 3 (100 psia, saturated liquid) draw a vertical line (constant enthalpy line) to intersect with the 5 psia horizontal line. This intersection defines state point 4 in the second tank. h4 = - 10 Btu=lbm, P4 = 5 psia Using state point 4 as reference on the P–h diagram, interpolate between the given curves for entropy (s) and for quality (x) to obtain the properties at state point

164

2 Chemical Thermodynamics

4. From the graph, carbon dioxide is a solid–vapor mixture at state point 4 with quality, x = 0.44, that is 44% vapor and with entropy, s4 = 0:03 Btu=lbm‐ ° R:

Practice Problem 2.7 Solution Determine the partial pressure of each component in the vapor phase using Raoult’s law (Eq. 2.36). PC6 H14 = xC6 H14 Psat C6 H14 = ð0:4Þð189 kPaÞ = 75:6 kPa PC8 H18 = xC8 H18 Psat C8 H18 = ð0:6Þð33:5 kPaÞ = 20:1 kPa Calculate the total pressure in the vapor phase using Dalton’s law (Eq. 2.18). n

Pi = PC6 H14 þ PC8 H18 = 75:6 kPa þ 20:1 kPa = 95:7 kPa

P= i=1

Calculate the mole fractions in the vapor phase using the relationship mole fraction = pressure fraction ðEq: 2:17Þ PC6 H14 75:6 kPa = = 0:79 P 95:7 kPa PC H 20:1 kPa yC8 H18 = 8 18 = = 0:21 P 95:7 kPa yC6 H14 =

Practice Problem 2.8 Solution The temperature of the liquid mixture can be determined by using the following trial and error procedure: 1. Assume a liquid temeperature, say an average value of the given NBPs (252 K). 2. Find the vapor pressures of the components at the assumed temperature. 3. Calculate the partial pressures of the components in the vapor phase by using Raoult’s law.

Solutions to Practice Problems

165

4. If the partial pressures add up to 101 kPa, then the assumed temperature is correct, if not repeat steps 1–3 until the pressures add up to 101 kPa. Subscript “1” represents propane and subcript “2” represents butane. Assume a temperature of 252 K  -21 °C. At -21 °C, the vapor pressures are sat Psat 1 = 0:236 MPa, and P2 = 0:043 MPa

Calculate the partial pressures using Raoult’s law (Eq. 2.36) and add them up. P = P1 þ P2

=

sat x1 Psat 1 þ ð1 - x1 ÞP2

= =

ð0:30Þð0:236 MPaÞ þ ð0:70Þð0:043 MPaÞ 0:1009 MPa ð ≃ 101 kPaÞ

The temperature of the liquid mixture is -21 °C ( 252 K).

Practice Problem 2.9 Solution Calculate the partial pressure of TCE in the vapor phase using Eq. 2.38. PTCE = H TCE cTCE = 1:20

kPa mol=m3

0:0001 mol=m3 = 0:00012 kPa

The vapor phase is an ideal gas mixture, where, Mole fraction TCE = pressure fraction TCE. From Eq. 2.17, yTCE =

PTCE 0:00012 kPa = = 1:185 × 10 - 6 P 101:3 kPa

Practice Problem 2.10 Solution Calculate the vapor pressure of chloroform at 327 K using the given Antoine’s equation.

166

2

Chemical Thermodynamics

1233 1233 = 4:21 = - 0:1012 T - 41 327 - 41 ) Psat CHCl3 = 0:7921 bar

log 10 Psat CHCl3 = 4:21 -

Calculate the liquid-phase activity coefficient of chloroform using Eq. 2.39, keeping in mind that for an azeotropic mixture the liquid and vapor compositions are equal, that is, yi = xi. y1 P J 1 x1 P1sat Ÿ

J1

P P1sat

1.013 bar 1.2789 0.7921 bar

Practice Problem 2.11 Solution The liquid compositions are xA = 0.80 and xB = 0.20. The temperature and vapor composition at atmospheric pressure can be determined by using the following trial and error procedure. 1. Assume a temperature. 2. Determine the vapor pressures of the components at the assumed temperature using the given Antoine’s equations: log Psat A = 5:37 -

1670 , T - 40:2

log Psat B = 6:21 -

2355 T þ 7:56

3. Determine the partial pressures of the components in the vapor phase by using the modified Raoult’s law: PA = γ A xA Psat A ,

PB = γ B xB Psat B

4. Check if PA + PB = 1.013 bar. 5. If yes, then the assumed temperature is correct. If not, repeat the preceding steps until the condition in step 4 is satisfied.

Solutions to Practice Problems

167

The results are summarized in the following table (temperature in K, pressures in bar). T(K) 350 351 352 351.5

Psat A (bar) 0.955 0.993 1.033 1.012

Psat B (bar) 0.417 0.438 0.457 0.448

PA (bar) 0.787 0.818 0.851 0.834

PB (bar) 0.166 0.174 0.182 0.178

PA + PB (bar) 0.953 0.992 1.033 1.012

Therefore, the temperature is 351.5 K. The vapor compositions can be determined from Eq. 2.17 (mole fraction = pressure fraction). yA =

PA 0:834 = 0:8241 = 1:012 P

yB =

PB 0:178 = 0:1759 = 1:012 P

Practice Problem 2.12 Solution Component A represents cyclohexane and Component B represents water. Calculate the mole fraction of cyclohexane in the vapor phase using Eq. 2.48. yA =

ð4:33Þð0:25Þ αAB xA = = 0:5907 1 þ ðαAB - 1ÞxA 1 þ ð4:33 - 1Þð0:25Þ

Calculate mole fraction of water in the vapor phase. yB = 1 - yA = 1 - 0:5907 = 0:4093

Practice Problem 2.13 Solution Subscript “1” represents propane, subscript “2” represents isobutane, and subscript “3” represents isopentane. The following procedure can be used for determining the dew point. 1. Assume a starting temperature. 2. Using the given pressure and the assumed temperature, determine Ki’s for all the components from the nomograph. 3. Calculate the sum of liquid mole fractions using the equilibrium relationship, yi = Kixi. 4. If the sum of liquid mole fractions is 1.0, then the assumed temperature is the dew point. If not, repeat steps 1 through 3 until convergance to dew point.

168

2

Chemical Thermodynamics

Assume TDP = 150 ° F as an intial guess. From the solution nomograph, corresponding to 50 psia and 150 °F, K1 = 7.2, K2 = 3.0, K3 = 0.88. Using Eq. 2.41, x1 þ x 2 þ x 3

y y1 y þ 2 þ 3 K1 K2 K3 0:20 0:30 0:50 = þ þ 7:2 3:0 0:88 = 0:7527ð ≠ 1:0Þ =

Try TDP = 130 ° F. From the nomograph, corresponding to 50 psia and 130 °F, K1 = 6.8, K2 = 2.5, K3 = 0.59. Using Eq. 2.41, x1 þ x2 þ x3 =

y y1 y 0:20 0:30 0:50 þ 2 þ 3 = þ þ = 0:9969ð ≃ 1:0Þ: K1 K2 K3 6:8 2:5 0:59

Therefore, the dew point is TDP = 130 ° F and the liquid composition is:

Solutions to Practice Problems

x1 =

169

0:20 0:30 0:50 = 0:0294, x2 = = 0:1200, x3 = = 0:8474 6:8 2:5 0:59

Practice Problem 2.14 Solution Subscript “1” represents n-hexane, subscript “2” represents n-heptane, and subscript “3” represents n-octane. The following procedure can be used for determining the total pressure and hence the vapor composition. 1. Assume a starting pressure. 2. Using the given temperature and the assumed pressure, determine Ki’s for all the components from the nomograph. 3. Calculate the sum of vapor mole fractions using the equilibrium relationship, yi = Kixi. 4. If the sum of vapor mole fractions is 1.0, then the assumed pressure is correct. If not, repeat steps 1 through 3 until convergance.

170

2

Chemical Thermodynamics

Try P = 20 psia as an intial guess. From the solution nomograph, corresponding to 20 psia and 80 °F, K 1 = 1:50, K 2 = 0:40, K 3 = 0:09 Using Eq. 2.41, y1 þ y2 þ y3

= K 1 x1 þ K 2 x2 þ K 3 x3 = 1:50 × 0:30 þ 0:40 × 0:30 þ 0:09 × 0:40 = 0:45 þ 0:12 þ 0:036 = 0:606ð ≠ 1:0Þ

Final trial: P = 16 psia. From the solution nomograph, corresponding to 16 psia and 80 °F, K 1 = 2:70, K 2 = 0:60, K 3 = 0:15 Using Eq. 2.41, y1 þ y2 þ y3 = K 1 x1 þ K 2 x2 þ K 3 x3 = 2:70 × 0:30 þ 0:60 × 0:30 þ 0:15 × 0:40 ,

= 0:81 þ 0:18 þ 0:06 = 1:05ð ≃ 1:0Þ

P = 16 psia and y1 = 0:81, y2 = 0:18, y3 = 0:06,

Practice Problem 2.15 Solution Convert the temperatures to absolute values. T 1 = 460 ° þ 228 ° F = 688 ° R

T 2 = 460 ° þ 250 ° F = 710 ° R

Calculate the enthalpy of vaporization on a molar basis. ΔH vapn: = ΔH vapn: × M = 960

Btu lbm × 18 = 17, 280 Btu=lbmol lbm lbmol

Solve Eq. 2.63 for the ratio of the vapor pressures.

Solutions to Practice Problems

ln

171

ΔH vapn: Psat 2 = Psat R 1

Psat 2 =e Psat 1

ΔH vapn: R

1 T1

1 1 T1 T2 - T1

2

=e

)

17280 Btu lbmol Btu lbmol‐ ° R

1:986

ð6881° R - 7101° RÞ

= 1:4798

sat Psat 2 = 1:4798P1 = 1:4798 × 20 psia = 29:60 psia

From the excerpt of the steam table shown, the saturation pressure at 250 °F is 30 psia which is in good agreement with the analytically calculated result.

Practice Problem 2.16 Solution The equilibrium concentration of phosphorus trichloride is already given. From the given information and the stoichiometric reaction equation, calculate the molar concentrations of chlorine and phosphorus pentachloride at equilibrium as shown. N PCl3,cons = N i,PCl3 - N eq,PCl3 = 0:30

mol mol - 0:15 = 0:15 mol=L L L

N Cl2,cons = N PCl3,cons = 0:15 mol=L N eq,Cl2 = N i,Cl2 - N Cl2,cons = 0:25

mol mol - 0:15 = 0:10 mol=L L L

N PCl5,prod = N PCl3,cons = 0:15 mol=L = N PCl5,eq From Eq. 2.64, the equilibrium constant is the ratio of the concentration of the product species raised to the power of the stoichiometric coefficient of each species to the concentration of the reactant species raised to the power of the stoichiometric coefficient of each species. In this case, phosphorus pentachloride is the product and

172

2 Chemical Thermodynamics

phosphorus trichloride and chlorine gas are reactants, all with stoichiometric coefficients of 1. Therefore, KC =

½PCl5  0:15 = = 10 ½PCl3 ½Cl2  0:15 × 0:10

Practice Problem 2.17 Solution Apply the equation for the equilibrium constant (Eq. 2.64) to the given reaction. KC

=

½C c ½Dd ) ½Aa ½Bb

KC

=

½NH3 2 ½N2 ½H2 3

) 0:0045

=

2:152 3:85½H2 3

Solve the preceding equation for [H2], the equilibrium concentration of hydrogen. ½H2  = 6:44

Practice Problem 2.18 Solution Assume that x mols of hydrogen, H2, is consumed before equilibrium is reached. Based on the stoichiometric equation and the consumption of x mols of hydrogen, x mols of carbon dioxide, CO2 are consumed, and x mols of water vapor, H2O, and x mols of carbon monoxide, CO, are produced. Using the initial concentrations and the consumptions, construct the “ICE” table as shown. Species concentration (mol/L) Initial Change Equilibrium

[H2] 0.013 -x 0.013 - x

[CO2] 0.016 -x 0.016 - x

[H2O] 0 x x

[CO] 0 x x

Apply the equation for the equilibrium constant (Eq. 2.64) to the given situation.

Solutions to Practice Problems

KC = KC = 0:08 =

173

½Cc ½Dd ) ½Aa ½Bb ½H2 O½CO ð xÞ ð xÞ ) 0:08 = ) ½H2 ½CO2  ð0:013 - xÞð0:016 - xÞ x2 ) x = 0:0032 mol=L ð0:013 - xÞð0:016 - xÞ

Therefore, the equilibrium concentrations are mol mol - 0:0032 = 0:0098 mol=L L L mol mol ½CO2  = 0:016 - 0:0032 = 0:0128 mol=L L L ½H2  = 0:013

½H2 O = ½CO = 0:0032 mol=L

Practice Problem 2.19 Solution Convert the temperature to its absolute value, T = 273 ° + 25 ° C = 298 K Substitute the known values into Eq. 2.70 to calculate ΔG. ½NH3 2 ½N2 ½H2 3

 ln ΔG = ΔG0 þ RT = - 16:4 ln

kJ kJ þ 0:008314 × 298K × mol mol  K

ð0:36Þ2 ð0:17Þð0:19Þ3

= - 4:728 kJ=mol To determine the required direction of the reaction, determine the value of the equilibrium constant, KC, using Eq. 2.71. ΔG0 = - RT ln K C ) KC = e

-

ΔG0 RT

=e

-

- 16:4 kJ mol 0:008314 kJ × 298 K molK

= 749:5

Compare the preceding value of KC with the reaction quotient at the given condition.

174

2

QC =

Chemical Thermodynamics

½NH3 2 ð0:36Þ2 = = 111:15 ½N2 ½H2 3 ð0:17Þð0:19Þ3

Since QC < < KC, the reaction has to proceed in the forward direction to increase the concentration of the products and to decrease the concentration of reactants.

References 1. Y. Cengel, M. Boles, Thermodynamics – An Engineering Approach, 9th edn. (McGraw Hill, New York, 2019) 2. N. Hall, First Law of Thermodynamics (NASA Glenn Research Center, 2021), Download from https://www.grc.nasa.gov/www/k-12/airplane/thermo1.html 3. G. Heany, The First Law of Thermodynamics – Open System (Control Volume), Slide Serve, (2012), Download from https://www.slideserve.com/garret/the-first-law-of-thermodynamics 4. P.E. Liley, Compressibility Factor (Thermopaedia, 2011) Download from https://www. thermopedia.com/content/645/ 5. I.H. Lone, The Alternative Formulation of Van’t Hoff Equation. Phys. Chem. Ind. J. 12(2), 1–6 (2017) 6. N.S. Nandagopal, Fluid and Thermal Sciences – A Practical Approach for Students and Professionals (Springer, Cham, 2022) 7. L.K. Nash, Elements of Chemical Thermodynamics, 2nd edn. (Addition Wesley, Reading, 2005) 8. B.J. Ott, J. Boerio-Goates, Chemical Thermodynamics – Principles and Applications (Elsevier Academic Press, New York, 2000) 9. A.P.H. Peters, Concise Chemical Thermodynamics, 3rd edn. (Taylor Francis Group, Boca Raton, 2010) 10. J.M. Smith, H. Van-Ness, M. Abbott, M. Swihart, Introduction to Chemical Engineering Thermodynamics, 9th edn. (McGraw Hill Engineering Series, New York, 2022)

Chapter 3

Fluid Mechanics and Momentum Transfer

3.1

Role of Fluid Mechanics in Chemical Engineering

Every unit operation conducted in the chemical process industry involves the transport and movement of fluids, which is essentially fluid mechanics [3]. Consider the unit operation of distillation. Here, the feed as well as the reflux needs to be conveyed to the distillation column and the products from the distillation unit need to be removed for storage/transport to another processing unit. The design of piping/ pumping systems used for fluid transport involves knowledge of fluid mechanics. The situation is similar in other unit operations in chemical engineering such as gas absorption, heat exchange, and filtration. Feed/product/recycle/bypass streams need to be moved between different units, which is essentially transport of fluids. Then, there are specialized fluid mechanics topics such as flow through packed beds, fluidized beds, flooding of columns which can be considered as being exclusively within the realm of chemical engineering. Control of chemical processes involves measurement and monitoring of process variables, of which flow rate is of prime importance. Fluid flow measurement using devices such as orifice meter requires knowledge and application of the Bernoulli’s principle from fluid mechanics. It is worth noting the unified approach of transport phenomenon encompassing the topics of momentum, heat, and mass transfer was largely developed by chemical engineers [3, 6]. Therefore, fluid mechanics is an essential arrow in the quiver of a chemical engineer.

3.2

Fluid Properties and Fundamentals

The important properties that characterize a fluid are density (ρ), specific weight (γ), specific gravity (SG), dynamic viscosity (μ), and kinematic viscosity (ν) [4, 5].

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 N. S. Nandagopal, Chemical Engineering Principles and Applications, https://doi.org/10.1007/978-3-031-27879-2_3

175

176

3

Fluid Mechanics and Momentum Transfer

Density: The density (ρ) of a fluid is its mass per unit volume. The units for density are lbm/ft3 (USCS) and kg/m3 (SI). The standard density values of water are 62.4 lbm/ft3 (in the range 45–65 °F) and 1000 kg/m3 (in the range 5 °C to 20 °C). Standard values of density of water ρw = 62.4 lbm/ft3 (USCS) ρw = 1000 kg/m3 (SI) Specific Weight: The specific weight (γ) of a fluid is the weight per unit volume of the fluid. Weight, W, is the force experienced by a body due to the gravitational acceleration, g. On earth, g = 32.2 ft/sec2 (USCS) and 9.81 m/s2 (SI). Units of specific weight are lbf/ft3 (USCS) and N/m3 (SI). Weight can be calculated from Newton’s second law by using g for acceleration, W = mg (SI units). However, the formula for weight in USCS requires the use of the conversion constant, gc, to obtain proper units for force, which is lbf. W=

32:2 lbm‐ft mg ðUSCSÞ, where gc = gc lbf‐ sec 2

The formulas for specific weight are given here.

W γ= = V γ=

mg gc V

=

m V

g ρg ðUSCSÞ = gc gc

W mg m g = ρg ðS IÞ = = V V V

The standard specific weight of water is 62.4 lbf/ft3 (in the range 45–65 °F) and 9810 N/m3 (in the range 5–20 °C). However, since the SI unit of pressure is usually kPa, it is preferable to use the value 9.81 kN/m3 as the standard specific weight of water. Standard specific weights of water γ w = 62.4 lbf/ft3 (USCS) and γ w = 9.81 kN/m3 (SI) Specific Gravity: Specific gravity (SG) is commonly used for liquids and is also known as relative density. The specific gravity (SG) of a liquid is the ratio of the density of the liquid to the standard density of water. It is also the ratio of the specific weight of the liquid to the standard specific weight of water. SG =

γ liquid ρliquid = ρwater,std γ water,std

ð3:1Þ

3.2

Fluid Properties and Fundamentals

177

Most oils and hydrocarbon liquids are lighter than water and have specific gravity less than 1.0 and the specific gravity of mercury (Hg), commonly used in manometers, is 13.6. Dynamic Viscosity: The viscosity of a fluid is a property that indicates the flow resistance of the fluid. A clear understanding of viscosity can be obtained by considering the definition of the absolute or dynamic viscosity (μ) of a fluid, which is defined as the ratio of the applied shear stress to the rate of shear deformation (velocity gradient) of the fluid. μ=

τ

ð3:2Þ

dv dy

In Eq. 3.2, τ, is the shear stress parallel to the direction of motion of the fluid. The shear stress is the applied force divided by the contact area of the fluid with the surface. The shear stress results in relative motion between different layers of the fluid. The layer adjacent to the surface has zero velocity to satisfy the “no-slip condition.” The velocity gradually increases in the y-direction perpendicular to the flow surface. This results in a fairly linear velocity profile from the surface to the outer edge of the boundary layer. The slope of the velocity profile is the velocity gradient, (dv/dy). For the same applied shear stress, the velocity gradient will be relatively small for high viscous fluids such as oils. In other words, the change in the fluid velocity will be small as we move away from the surface. Water has a relatively low viscosity because of the high velocity gradient in the direction perpendicular to the surface. The units of dynamic viscosity can be obtained by substituting the units for the physical quantities on the right side of Eq. 3.2.

μ

τ dv dy

lbf 2 lbf‐ sec    ft ft= sec ft2 ft

μ

τ dv dy

N 2 Ns m  2   m=s m m

lbm‐ft ðsec Þ sec 2 lbm  2 ft‐ sec ft

32:2

kg‐m ðsÞ s2 kg  ms m2

Since N/m2 is Pascal (Pa), a unit of pressure, the units of dynamic viscosity in the SI system can be represented as Pa.s. In industrial practice, the units of centipoise (cP) are commonly used for μ, the dynamic viscosity. Poise = dyne.s/cm2 1 cP = 10-2 Poise

178

3

Fluid Mechanics and Momentum Transfer

Conversion factors 1 lbm/ft-sec = 1.4881 kg/ m.s = 1.4881 N.s/m2 (Pa.s) 1 lbf-sec/ft2 = 32.2 lbm/ft-sec = 47.88 N.s/m2 (Pa.s) 1 cP = 0.001 N.s/m2, (Pa.s), (kg/m.s) 1 cP = 0.000021 lbf-sec/ft2 = 0.000672 lbm/ft-sec Example 3.1 An experimental apparatus used for measuring dynamic viscosity consists of two concentric cylinders. The outer radius of the inner cylinder is 50 cm and the inner radius of the outer cylinder is 51.5 cm. The liquid to be studied is placed in the annular space between the cyclinders. In this particular instance, the inner cylinder is rotated at 12 rpm, while the outer cyclinder is stationary. The torque required to rotate the inner cylinder is measured to be 0.925 N.m. The length of each cylinder is 8 cm. A. Derive an equation for the torque required to rotate the inner cylinder. B. Calculate the dynamic viscosity of the liquid using the equation obtained in part (A). Solution A schematic diagram of the apparatus is shown here.

A. Since the difference between Ri and Ro is very small, the average radius can be used for calculations. The force experienced by the inner cylinder is the product of the shear stress and the surface area. F = τAs = τ 2πRavg L The torque required is the force multiplied by the average radius and in this case the dynamic viscosity is the ratio of the shear stress to the velocity gradient in the radial direction. The linear velocity is the product of the angular velocity and the radius (v = rω). T = FRavg = τ 2πRavg L Ravg = μ

dv dr

2πR2avg L = μ

Δv Δr

2πR2avg L

3.2

Fluid Properties and Fundamentals

179

B. Solve the preceding equation for the dynamic viscosity and subsequently substitute all the known values. μ=

T 2πR2avg L

=

Δv ΔR

T 2πR2avg L

Ro ωo - Ri ωi Ro - Ri

Ro = 51:5 cm  0:515 m, Ri = 50 cm  0:50 m, L = 8 cm  0:08 m Ravg =

Ri þ Ro 0:50 m þ 0:515 m = = 0:5075 m 2 2 2π 12 Rev: = 75:4=min and Rev: min ωo = 0 since the outer cylinder is stationary

ωi = 2πN i =

Substitute all the known values into the equation for dynamic viscosity. μ= 2πR2avg L =

T Ro ωo - Ri ωi Ro - Ri 0:925 N:m

75:4 min ð2π Þð0:5075 mÞ ð0:08 mÞ 0:515 m - 0:50 m 0 - 0:50 m ×

2

= 0:00284 N:min=m2 Note: Since the inner cup is rotating and the outer cup is stationary, the velocity decreases with increase in r and therefore the negative sign can be ignored. Convert the result into the standard units for dynamic viscosity (N.s/m2) μ = 0:00284

N  min m2

60 s min

= 0:1704 N  s=m2

Kinematic Viscosity: Kinematic viscosity (ν), is the ratio of absolute viscosity to the density of the fluid. ν=

μ ρ

ð3:3Þ

180

3

Fluid Mechanics and Momentum Transfer

The units of kinematic viscosity can be obtained by substituting the units for dynamic viscosity and density in Eq. 3.3. lbm ft2 μ ft‐ sec   ðUSCSÞ lbm ρ sec ft3 kg μ m  s m2  ν  ðS IÞ kg ρ s m3

ν

A unit of kinematic viscosity that is frequently used in the industry is centistokes (cSt) which is based on the unit Stoke, which is defined as cm2/s. Conversion factors 1 m2/s = 10.764 ft2/s 1 cSt = 0.000001 m2/s = 0.000011 ft2/s

3.3

Fluid Statics and Manometers

The topic of Fluid Statics examines the theory and applications of fluids at rest. The fundamental concepts in fluid statics are static pressure, pressure head, and pressure difference [4, 5]. Differential manometers are extensively used in chemical engineering applications for measuring pressure differences.

3.3.1

Static Pressure and Pressure Head

Consider a fluid in a container (Fig. 3.1). The force that acts on the bottom surface of the container is the weight of the fluid. The weight of the fluid is the volume of the fluid multiplied by the specific weight of the fluid. The pressure or pressure intensity Fig. 3.1 Static pressure due to a fluid column

3.3

Fluid Statics and Manometers

181

of the fluid on the surface is defined as the force per unit area. The pressure intensity acts uniformly on the surface. P=

γ ðAhÞ F W γV = γh = = = A A A A

Therefore, the static pressure due to a fluid column is the product of the specific weight of the fluid and the depth of the fluid from the free surface. The depth of the fluid column, h, is known as the pressure head. P = γh

ð3:4Þ

Units of pressure: Since pressure is force per unit area, it has units of lbf/ft2 (USCS), and N/m2 (SI system). However, the more commonly used units in practice are lbf/in2 (psi), and kPa, where N/m2 is also known as Pascal (Pa). Another commonly used unit for pressure is “bar” and 1 bar = 105 N/m2 = 100 kPa. Pressure increases with increase in depth and decreases with increase in elevation. The static pressure calculated by Eq. 3.4 is the pressure relative to atmospheric pressure and it is also known as gage pressure since the pressure indicated by a pressure gage attached to the container at any depth is the static pressure due to the fluid column. In fluid mechanics, the term “pressure” always refers to gage pressure, unless specified otherwise. In thermodynamics, the pressure used is generally the absolute pressure, which is the sum of the atmospheric pressure and the gage pressure. Thus, absolute pressure is the pressure of the fluid relative to zero pressure or perfect vacuum and gage pressure is the pressure of the fluid relative to the pressure of the surroundings (usually the local atmospheric pressure). Note: In fluid mechanics, gage pressures are usually used whereas in thermodynamics, absolute pressures are usually used. Conversion factors: 1 psi = 6.895 kPa = 0.0689 bar and, 14.7 psi = 101.3 kPa Atmospheric pressure (Patm) is the pressure exerted by local atmospheric air and is also known as barometric pressure, since it is measured by a mercury barometer. The value of this depends on the local elevation. The equivalent values of atmospheric pressure at sea level (zero elevation) are given here. Standard atmospheric pressure at sea level: 1 atm = 760 mm Hg = 29.92 in Hg = 14.7 psi = 101.3 kPa = 1.013 bar The equivalent pressure head of water column for atmospheric pressure is derived here.

hwater =

P = γ water

144 in2 ft2 lbf 62:4 3 ft

lbf in2

14:7

= 33:92 ft of water

182

3

hwater =

P = γ water

101:3 kPa  9:81

kN m3

Fluid Mechanics and Momentum Transfer

kN m2

= 10:33 m of water

Pressure equivalents: 1 atm = 14.7 psi = 101.3 kPa = 33.92 ft of water = 10.33 m of water 1 psi = 2.31 ft of water, 1 kPa = 0.1020 m of water The absolute pressure of a system in vacuum is lower than the atmospheric pressure. Vacuum gages measure vacuum relative to atmospheric pressure. Vacuum gage readings must be subtracted from atmospheric pressure to obtain the absolute pressure.

3.3.2

Differential Manometers

Differential manometers are devices used in measuring the pressure difference between two points [4–6]. The most common device is the “U-–tube manometer” shown in Fig. 3.2. The two legs of the manometer are connected to pressure taps upstream and downstream of the pipe in which a fluid is flowing. The manometer consists of a manometric fluid, which is usually heavier than the fluid flowing in the pipe.

Fig. 3.2 U-tube differential manometer

3.3

Fluid Statics and Manometers

183

Mercury (Hg) is commonly used as a manometric fluid in water pipes. For air ducts, water is commonly used as a manometric fluid. Because of the pressure drop across the pipe, P1 > P2 and the equation for the pressure difference can be obtained by using the principles of static pressure head (P = γh). Determine the pressures at points 3 and 4 by using the principle that pressure increases as we move down a fluid column and pressure decreases as we move up a fluid column. P3 = P1 þ γ f ðhf þ hm Þ P4 = P2 þ γ f hf þ γ m hm Since reference points 3 and 4 are at the same level in the same fluid, P3 = P 4 Substitute for P3 and P4 from the preceding equations. P1 þ γ f ðhf þ hm Þ = P2 þ γ f hf þ γ m hm Solve the preceding equation for the pressure difference and simplify. ΔP = P1 - P2 = hm ðγ m - γ f Þ

ð3:5Þ

Multiply and divide the right-hand side of Eq. 3.5 by γ f. ΔP = P1 - P2 = γ f hm

γm -1 γf

If the flowing fluid is water, from the definition of specific gravity (Eq. 3.1), SGm =

γm γf

Therefore, ΔP = P1 - P2 = γ f hm ðSGm- 1Þ

ð3:6Þ

Thus, the pressure difference is proportional to the manometer reading, hm, and it can be calculated by using either Eq. 3.5 (general equation applicable for flow of any fluid) or Eq. 3.6 (applicable only for flow of water).

184

3

Fluid Mechanics and Momentum Transfer

Example 3.2

A mercury (SG = 13.6) manometer is used in determining the pressure drop across the inclined water pipe in a vertical plane as shown in the figure. Determine the pressure difference PA - PB in the inclined pipe shown in the figure. Solution Locate the reference points for calculations as shown in the figure.

The pressures are equal at the same horizontal level in the same fluid. Therefore, P1 = P2

3.4

Fluid Dynamics

185

Determine P1 and P2 by using the static pressure equation (Eq. 3.4). P1 = PA þ γ w hA - 1 = PA þ 9:81

kN m3

15 mm ×

1m 1000 mm

= PA þ 0:1471 P2 = PB þ γ w hB - 3 þ γ m h3 - 2 = PB þ 9:81

kN m3

þ 13:6 × 9:81

30 mm ×

kN m3

1m 1000 mm

27 mm ×

1m 1000 mm

= PB þ 3:8965 P1 = P2 ) PA þ 0:1471 = PB þ 3:8965 ) PA - PB = 3:7494 kPa Explanation of Units : γh 

3.4

kN m3

ðmÞ  kN m2  kPa

Fluid Dynamics

Fluid dynamics is the study of fluids in motion. Fluid dynamics includes flow through entities such as pipes, ducts, and open channels [3–5]. Typically, the focus is on the pressure drop due to flows in different entities. The flow of fluids also generates forces on objects such as pipe bends. The energy generated by fluids due to flow over turbine blades is harnessed for power generation. The study of flow through packed beds, flow through equipment such as distillation columns, and fluidized beds are of specific relevance in chemical engineering. The mass flow rate of the fluid usually forms the basis of all calculations in fluid dynamics. Since mass of a species is always conserved, the mass flow rate of the fluid remains constant in the flow field for steady flow.

3.4.1

Conservation of Mass – Continuity Equation

For steady flow of a fluid, the mass flow is constant. The Law of Conservation of Mass states that the mass flow into a control volume is equal to the mass flow out of the control volume. The typical units for mass flow rates are pound mass per hour (lbm/hr) and kilogram per second (kg/s).

186

3

Fluid Mechanics and Momentum Transfer

Fig. 3.3 Continuity equation

Conversion factor: 1 kg/s = 7937 lbm/hr Consider the section of the pipe shown in Fig. 3.3. The following nomenclature is used: m_ = mass flow rate of the fluid Q = volume flow rate of the fluid ρ = density of the fluid A = area of cross section of pipe v = average velocity in the pipe The mass flow into the section is the same as the mass flow out of the section, m_ 1 = m_ 2 = m_ The mass flow rate is the volume flow rate times the density of the fluid and the volume flow rate is the cross section area of the pipe times the average velocity of the fluid in that section. Therefore, m_ = Q1 ρ1 = Q2 ρ2

ð3:7Þ

m_ = A1 v1 ρ1 = A2 v2 ρ2

ð3:8Þ

Equation 3.8 is the most general form of the continuity equation. Due to pressure drop, the pressure at section 2 is less than the pressure at section 1. For gases, density is directly proportional to pressure and the densities at sections 1 and 2 are not equal because of the pressure drop. Since the density in the flow field is not constant, the flow of gases is described as compressible flow. The density of a liquid is constant and does not vary significantly with pressure. Liquids are essentially incompressible. Since ρ1 = ρ2 for liquids, the density terms cancel out in Eqs. 3.7 and 3.8. Hence, the continuity equation for liquids can be written in terms of volume flow rate and mass flow rate as: Q1 = Q2 = Q = A1 v1 = A2 v2 = Av

ð3:9Þ

m_ = ρ1 A1 v1 = ρ2 A2 v2 = ρAv = ρQ

ð3:10Þ

In incompressible flow, the volumetric flow rate of a liquid remains constant in the flow field. The volumetric flow rate has units of cubic meters per second (m3/s), and cubic feet per second (ft3/sec). In practice, the unit of gallons per minute (gpm) is widely used for the flow of liquids and the unit of cubic feet per minute (cfm) for flow of gases.

3.4

Fluid Dynamics

187

Conversion factors: 1 m3/s = 35.315 ft3/sec (cfs) 1 ft3/sec = 449 gpm 1 m3/s = 2119 cfm Example 3.3 In a shell and tube heat exchanger, being used in a process facility, the mass flow rate of the tube side fluid increases by 15%. Determine the percent increase in the ID (inside diameter) of the tubes required to maintain the same velocity as before. Solution Subscript “1” represents the original situation prior to the increase in the mass flow rate and subscript “2” represents the situation after the increase in the mass flow rate. From Eq. 3.10,

Percent increase in ID =

3.4.2

D2 - D1 D2 - 1 100 = ð1:0724 - 1Þ100 = 7:24% 100 = D1 D1

Laminar Flow and Turbulent Flow Through Pipes

Laminar flow can be described as smooth and streamlined flow of fluid. In laminar flow, fluid particles remain in the same streamline and do not move to adjacent streamlines. As a result, there is no exchange of momentum between adjacent fluid layers. The velocity distribution in a pipe is symmetrical about the center line of the pipe. The velocity at the pipe wall is zero. The velocity gradually increases away from the wall and reaches a maximum value at the center line of the pipe (Fig. 3.4). In turbulent flow, there is mixing of fluid particles from different streamlines. As a result, the faster moving fluid particles situated near the center of the pipe will transfer some of their momentum to the slower moving fluid particles situated near the wall of the pipe. Hence, the velocity profile for turbulent flow will be more uniform and flatter as shown in Fig. 3.5. It can be observed from the velocity profile for turbulent flow that the average velocity is quite close to the maximum velocity. In some cases, the average velocity can be as high as 80% of the maximum velocity at the center line of the pipe.

188

3

Fluid Mechanics and Momentum Transfer

Fig. 3.4 Velocity profile for laminar flow in pipes Fig. 3.5 Velocity profile for turbulent flow in pipes

3.4.3

Reynolds Number

The Reynolds number is a dimensionless number used in fluid dynamics. It represents the ratio of inertial forces to viscous forces [5]. The inertial force is responsible for the motion of the fluid due to the momentum it carries and therefore it is proportional to the velocity and density of the fluid. The viscous force tends to resist the flow of the fluid and it is proportional to the viscosity of the fluid. The Reynolds number is represented by Re and can be calculated using the following formula. Re =

Dvρ μ

ð3:11Þ

In Eq. 3.11, D is the inside diameter (ID) of the pipe, v is the average velocity of the fluid, ρ is the density of the fluid, and μ is the dynamic viscosity of the fluid. From Eq. 3.3, the ratio of dynamic viscosity to the density of the fluid is the kinematic viscosity of the fluid, ν. Therefore, the formula for the Reynolds number can also be written in terms of kinematic viscosity as shown in Eq. 3.12. Re =

Dvρ Dv Dv = μ = μ ν ρ

ð3:12Þ

3.4

Fluid Dynamics

189

Extreme care must be taken to ensure consistency of units while calculating the Reynolds number. All the units in the formula must cancel out leaving a dimensionless result. The Reynolds number is used as a criterion for determining if the flow in a pipe is laminar or turbulent. Laminar flow occurs in a pipe if the Reynolds number, Re, is less than 2100. Generally, the flow is fully turbulent if Re > 10,000. The flow is in the “transient zone” if 2100 < Re < 10000. Example 3.4 Gasoline (SG = 0.92, μ = 0.59 cP) flows through a 52.5 mm ID pipe at a mass flow rate of 0.30 kg/s. Determine the Reynolds number for the flow of gasoline. Solution Calculate the density of gasoline by using the definition of specific gravity, SG (Eq. 3.1). SGgasoline =

ρgasoline ) ρwater,std

ρgasoline = SGgasoline × ρwater,std = ð0:92Þ 1000

kg = 920 kg=m3 m3

Convert the pipe ID to meters. D = 52:5 mm ×

1m = 0:0525 m 1000 mm

Calculate the velocity of gasoline by using the continuity equation (Eq. 3.10) v=

0:30 kgs m_ = 0:1506 m=s = ρA 920 mkg3 π4 ð0:0525 mÞ2

Convert the dynamic viscosity of gasoline to kg/m.s μ = 0:59 cP × 0:001

kg=m:s = 0:00059 kg=m:s cP

Calculate the Reynolds number using Eq. 3.11. Re =

kg m Dvρ ð0:0525 mÞ 0:1506 s 920 m3 = 12329 = μ 0:00059 kg m:s

190

3.4.4

3

Fluid Mechanics and Momentum Transfer

Friction Head Loss and Pressure Drop for Fluid Flow in Pipes

Fluid flow is always accompanied by a loss in pressure in the direction of flow. A fluid always flows from a point of higher pressure to a point of lower pressure. Any fluid flowing in a pipe has to overcome the resistance due to pipe friction. This results in a loss in energy of the fluid, which can be represented as a loss in static head corresponding to the pressure drop across the section of the pipe. The pressure drop and the friction head loss can be related by the following equation based on Eq. 3.4 presented earlier. ð3:13Þ

ΔP = γhf

Useful conversion factors between pressure and static head are presented again here for reference. Conversion factors: 1 atm = 14.7 psi = 101.3 kPa = 33.92 ft of water = 10.33 m of water

3.5

Calculating the Friction Head Loss for Pipe Flow

Intuitively, it can be hypothesized that the friction head loss due to fluid flow in a pipe would be directly proportional to the roughness of the pipe surface, the length of the pipe section, and the velocity of flow and it would be inversely proportional to the diameter of the pipe. Thus, the head loss due to pipe friction can be calculated using the Fanning equation [3, 4, 6] for friction head loss (Eq. 3.14). hf = ð4f Þ

L D

v2 2g



2fLv2 gD

ð3:14Þ

The following nomenclature is used in the preceding equation. hf = friction head loss (ft, m) f = Fanning friction factor (dimensionless) L = length of the pipe (ft, m) D = internal diameter of the pipe (ft, m) v = velocity of flow (ft/sec, m/s) g = acceleration due to gravity (32.2 ft/sec2, 9.81 m/s2) Note that the term (L/D) is dimensionless, and the term (v2/2g) is known as the “velocity head” and it has the units of ft or m.

3.5

Calculating the Friction Head Loss for Pipe Flow

3.5.1

191

Fanning Friction Factor

From the Fanning equation for friction head loss (Eq. 3.14), it is clear that the friction head loss is directly proportional to the Fanning friction factor. The Buckingham Pi Theorem in Dimensional Analysis states that the number of π independent dimensionless parameters required to characterize any flow phenomenon is the total number of variables (n) influencing the flow minus the number of fundamental dimensions (r), that is, π dim:less groups = n - r

ð3:15Þ

From the study of fluid flow through pipes, it can be intuitively hypothesized that the Fanning friction factor, f, will be a function of the following five variables: the roughness of the pipe surface (ε), the inside diameter of the pipe (D), the velocity of the fluid (v), the density of the fluid (ρ), and the dynamic viscosity of the fluid (μ). The three fundamental variables are mass, length, and time. Therefore, n = 5 and r = 3. Substitute the preceding results into the equation describing the Buckingham Pi theorem (Eq. 3.15). π dim:less groups = n - r = 5 - 3 = 2 The two dimensionless parameters needed to study the functional dependence of friction factor are Reynolds number (Re) and the relative roughness (ε/D). The Reynolds number can be calculated using Eq. 3.11 or Eq. 3.12 and the relative roughness can be calculated using Eq. 3.16. Relative roughness, r =

ε D

ð3:16Þ

Most of the piping in the industry is commercial steel and pipe roughness, ε, for commercial steel is 0.00015 ft (in SI Units, ε = 0.046 mm). The Fanning friction factor can be determined using Reynolds number and relative roughness as parameters from the Fanning friction factor chart shown in Fig. 3.6. The details on the procedure to determine the Fanning friction factor are explained in the solution to example problems. Darcy Friction Factor: In mechanical engineering practice and literature, the Darcy friction factor is more commonly used rather than the Fanning friction factor [5]. The Darcy friction factor is four times the Fanning friction factor. f Darcy = 4 × f Fanning or

fD =4×fF

ð3:17Þ

192

3

Fluid Mechanics and Momentum Transfer

Fig. 3.6 Fanning friction factor chart. (Generated by the author, N.S. Nandagopal, P E)

The Darcy equation for calculating the friction head loss (Eq. 3.18) is based on the Darcy friction factor. hf = f D

L D

v2 L = ð4f F Þ 2g D

v2 2g

ð3:18Þ

Example 3.5 Using the same data as in Example 3.4 and assuming a smooth pipe surface, determine A. the Fanning friction factor. B. the pressure drop per meter length of the pipe. Solution The following data and results from Example 3.4 are reproduced here for reference. D = 0:0525 m, ρ = 920 kg=m3 , v = 0:1506 m=s μ = 0:00059 kg=m:s,

Re = 1:23 × 104 , SG = 0:92

3.5

Calculating the Friction Head Loss for Pipe Flow

193

A. Using the Reynolds number and the curve for a smooth pipe, determine the Fanning friction factor from Fig. 3.6 as shown.

From the figure shown, the Fanning friction factor is f = 0.0063. B. Calculate the friction head loss using Eq. 3.14. 2fLv2 ð2Þð0:0063Þð1 mÞ 0:1506 ms hf = = gD 9:81 sm2 ð0:0525 mÞ

2

= 0:00055 m

Calculate the specific weight of gasoline using Eq. 3.1. SGgasoline =

γ gasoline ) γ water,std

γ gasoline = SGgasoline × γ water,std = ð0:92Þ 9:81

kN = 9:025 kN=m3 m3

Calculate the pressure drop using Eq. 3.4. ΔP = γhf = 9:025

kN ð0:00055 mÞ = 0:005 kPa m3

Example 3.6 Assuming highly turbulent flow (Re>107), determine the percent increase in pressure drop when the inside diameter of the pipe is halved from 300 mm to 150 mm. The pipe roughness remains constant at 0.05 mm. All other flow parameters remain the same. Solution Subscript “1” represents the original situation prior to the decrease in the diameter and subscript “2” represents the situation after the decrease in the diameter. Derive an expression for the ratio between the velocities using the continuity equation (Eq. 3.9).

194

3

Fluid Mechanics and Momentum Transfer

Q A1v1 A2 v2 v2 v1

A1 A2

4 4

D12 D22

D1 D2

2

2

D1

4

0.5 D1

Derive an expression for the ratio between the pressure drops using Eqs. 3.13 and 3.14 with all the parameters being the same except the diameters, velocities, and the friction factors.

2 f 2 L2 v 22 P2 P1

hf 2 hf 1

g D2 2 1

2 f1 L1 v

f2 f1

v2 v1

2

D1 D2

g D1 From Fig. 3.6, the curves for the friction factor show a constant value for f in the region of highly turbulent flow (high Reynolds number) for a given value of pipe roughness. Hence, it is reasonable to assume that the Fanning friction factor will be a function of only the relative roughness in this situation. Calculate the relative roughness of the two pipes using the given data. ε 0:05 mm ¼ ¼ 0:00017 D1 300 mm ε 0:05 mm r2 ¼ ¼ ¼ 0:00033 D2 150 mm

r1 ¼

Assume Re = 107. Using this value of Re and the calculated values of r1 and r2 as parameters, determine the Fanning factors, f1 and f2, from the Fanning friction chart as shown.

Substitute the results obtained for all the ratios into the equation for the ratio between pressure drops. 0:0038 ΔP2 ð4Þ2 ð2Þ ¼ 36:85 ¼ 0:0033 ΔP1 Percent increase in pressure drop is

3.5

Calculating the Friction Head Loss for Pipe Flow

195

Table 3.1 Friction factor formulas Formula f= f= f=

16 Re 0:079 Re 0:25 0:046 Re 0:20

ð3:19Þ ð3:20Þ

Remarks Valid only for laminar flow

3 × 103 < Re < 105

Blausius equation

ð3:21Þ

Taitel–Dukler equation

p1 4f

= - 1:80 log

p1 4f

= 1:14 - 2 log r

p1 = - 2:0 log 4f

p1 = - 1:80 log 4f

ΔP = ð

3.5.2

Constraints Re < 2100

ð3:22Þ

6:9 Re

r 3:70 6:9 Re

ð3:23Þ þ

2:51 p Re f

r þ 3:7

ð3:24Þ ð3:25Þ

Re > 2100, r = 0

Smooth pipe

Re >

Re > 2100

Fully turbulent flow, rough pipes Colebrook equation

Re > 2100

Modified colebrook equation

3500 r

ΔP2 - ΔP1 ΔP Þ100 = ð 2 - 1Þ100 = ð36:85 - 1Þ100 = 3585% ΔP1 ΔP1

Equations for Calculating Friction Factors

The Fanning friction factor can also be calculated by using formulas presented in this section (Table 3.1) along with the corresponding equation numbers. Before using any formula, it is important to verify that the constraints associated with the formula are satisfied. Note: The formulas presented in Eqs. 3.22, 3.23, 3.24, and 3.25 were developed based on the Darcy friction factor, which is four times the Fanning friction factor ( fD = 4fF). The subscripts for the friction factors have been dropped and all the formulas have the coefficient 4 multiplying the Fanning friction factor and they can be further simplified if desired. Example 3.7 150 kg/s of water flows through a 250 mm DN commercial steel pipe (ID = 254 mm, ε = 0.05 mm). The properties of water at the flow temperature are: ρ = 1000 kg=m3 and μ = 1 cP: A. Determine the Reynolds number and hence the Fanning friction factor from the friction factor chart. B. Calculate the Fanning friction factor using the following equations (presented in Table 3.1) after checking their validity. (a) (b) (c) (d) (e)

Blausius equation Taitel–Dukler equation Equation for fully turbulent flow, rough pipes Colebrook equation Modified Colebrook equation

196

3

Fluid Mechanics and Momentum Transfer

C. Comment on the results. Solution Convert the pipe ID to m. D=

254 mm = 0:254 m 1000 mm m

Calculate the velocity of water by using the continuity equation (Eq. 3.5). kg

v=

150 s m_ = 2:96 m=s = kg π ρA 1000 m3 4 ð0:254 mÞ2

Calculate the Reynolds number (Eq. 3.6) and the relative roughness (Eq. 3.16). Re =

kg Dvρ ð0:254 mÞ 2:96 ms 1000 m3 = 7:52 × 105 = Ns 0:001 μ ð1 cPÞ 1 cPm2

r=

ε 0:05 mm = = 0:0002 D 254 mm

A. Using the Reynolds number and relative roughness as parameters, determine the Fanning friction factor from Fig. 3.6 as shown in the accompanying figure.

From the friction factor chart, f = 0:0038, as shown. B. (a) Blausius equation (Eq. 3.20) In this case, since Re = 7.52 × 105 > 105, the criterion for using the Blausius equation is not satisfied.

3.5

Calculating the Friction Head Loss for Pipe Flow

197

(b) Taitel–Dukler equation (Eq. 3.21) In this case, since Re = 7.52 × 105 > 105, the criterion for using the Taitel–Dukler equation is satisfied. 0:046 0:046 = Re 0:20 7:52 × 105

f=

0:20

= 0:0031

(c) Equation for fully turbulent flow, rough pipes (Eq. 3.23). In this case, the criterion for using the fully turbulent flow equation is not 7 3500 3500 satisfied. Calculate 3500 r = 0:0002 = 1:75 × 10 . Therefore, Re < r , and the criterion for using Eq. 3.23 is not satisfied. (d) Colebrook equation (Eq. 3.24) Calculate the Fanning friction factor using the Colebrook equation by trial and error. Try the value, f = 0.0037 1 r 2:51 p = - 2 log p þ 3:7 Re 4f 4f Colebrook Equation 1 1 p =p = 8:220 4f 4 × 0:0037 Value of Left Hand Side Calculate the value of the right-hand side of the Colebrook equation and compare with the value obtained for the left hand side. - 2log

r 2:51 þ p 3:7 Re 4f

= - 2log

0:0002 2:51 p þ 3:7 7:52 × 105 4 × 0:0037

= 8:18 Since the calculated values on both sides of the Colebrook equation are in close agreement, we can conclude that the Fanning friction factor obtained from the Colebrook equation would be f = 0:0037: (e) Modified Colebrook equation (Eq. 3.25). The fanning friction factor can be explicitly calculated using the Modified Colebrook equation as shown here.

198

3

1 6:9 r p = - 1:8 log þ Re 3:7 4f = - 1:8 log

Fluid Mechanics and Momentum Transfer 1:11

6:9 0:0002 þ 3:7 7:52 × 105

1:11

= 8:223 Solve the preceding equation for f, f = 0:0037 . C. Comment: The results from the Friction factor chart and those from the valid equations are in good agreement.

3.5.3

Hagen–Poiseuille Equation

For laminar flow, it can be shown that the Fanning friction factor is a linear function of the Reynolds number as per Eq. 3.26. f=

16 Re

ð3:26Þ

Substitute the preceding relationship into Eq. 3.14. v2 32μLv 32 L v2 = ) hf,Lam = Dvρ D g g ρgD2 ð3:27Þ μ 32μLv ρg ) ΔPLam =γhf,Lam =γ , γ =ρg ðSIÞ,andγ = gc ρgD2

hf,Lam =

16 2fLv2 =2 gD Re

L D

Combine the preceding results to obtain the following equations (in different systems of units) for calculating the pressure drop for laminar flow in pipes (Re < 2100). 32μLv ðSI, 3:28aÞ D2 32μLv ðUSCS, 3:28bÞ ΔP = gc D2 ΔP =

Equation 3.28(a, b) is known as the Hagen–Poiseuille equation.

3.6

Flow Through Non-circular Cross Sections

199

Example 3.8 The pressure drop across 8 m of a 500 mm DN (ID = 478 mm) oil (SG = 0.85) pipeline is measured to be 250 Pa. The dynamic viscosity of the oil at the flow temperature is 237 cP. Assuming laminar flow, determine the mass flow of the oil in kg/s. Verify the validity of the assumption by calculating the Reynolds number. Solution Rearrange the Hagen–Poiseuille equation (Eq. 3.28a) to calculate the velocity of the oil.

ΔPLaminar D2 = v= 32μL

250

N m2

478 mm ×

32 237 cP ×

2 1m 1000 mm

Ns m 2 ð8 m Þ cp

= 0:941 m=s

0:001

Verify if the flow is laminar by calculating the Reynolds number using Eq. 3.11.

Dvρ Re = = μ

ð0:487 mÞ 0:941

m s

0:237

0:85 × 1000

kg m3

Ns m2

= 1644 Note: Substitute kg.m/s2 for Newton (N) to get the units to cancel out. Since Re < 2100, the flow is laminar. Calculate the mass flow rate of oil by using the continuity equation (Eq. 3.8). m_ = ρAv = 850

kg m3

π ð0:487 mÞ2 4

0:941

m s

= 149 kg=s

3.6

Flow Through Non-circular Cross Sections

Fluid flow often occurs in non-circular sections such as in the annular region (the region between two concentric pipes) of a double pipe heat exchanger. Flow through non-circular sections are handled by using the “Hydraulic Diameter,” represented by the symbol DH. The diameter, D, in circular pipe formulas is replaced by DH for flow through non-circular sections. For example, the equivalent formula for Reynolds number for flow through a non-circular section is,

200

3

Re =

Fluid Mechanics and Momentum Transfer

DH vρ DH v = μ ν

ð3:29Þ

The hydraulic diameter is based on the concept of equivalent hydraulic radius (RH) being equal to the cross section area (A) divided by the wetted perimeter (P). The wetted perimeter is the total length of the duct in contact with the fluid. The following result is obtained by applying this concept to a circular pipe. RH =

π D2 D A = = 4 πD 4 P

The hydraulic diameter can be calculated by using Eq. 3.30. DH = 4RH = 4

A Cross SectionArea =4 P Wetted Perimeter

ð3:30Þ

Example 3.9 A double pipe heat exchanger is constructed by using a 12 in nominal (ID = 11.94 in) outer steel pipe and a 6 in nominal inner steel pipe (OD = 6.63 in and ID = 6.06 in). Hot oil (SG = 0.77, ν = 14.4 cSt) flows in the annular region at a mass flow rate of 250 lbm/sec. The length of the heat exchanger is 15 ft. Calculate the pressure drop of the oil in psi. Solution Determine the hydraulic diameter of the annular region using Eq. 3.30. Dio represents the inner diameter of the outer pipe and Doi represents the outer diameter of the inner pipe. The area of the annular region is the area between the inner surface of the outer pipe and the outer surface of the inner pipe. The wetted perimeter of the annular region is the sum of the circumferences of the outer surface of the inner pipe and the inner surface of the outer pipe. Convert the given diameters to feet. Doi =

6:63 in 12 in 1 ft

= 0:5525 ft

A =4 DH = 4 P

Dio =

11:94 in 12 in 1 ft

= 0:9950 ft

π Dio 2 - Doi 2 4 π ðDio þ Doi Þ

= Dio - Doi = 0:9950 ft - 0:5525 ft = 0:4425 ft Calculate the density of oil by using the definition of specific gravity (Eq. 3.1)

3.6

Flow Through Non-circular Cross Sections

201

ρoil = ðSGÞðρwater,std Þ = ð0:77Þ 62:4

lbm = 48:05 lbm=ft3 ft3

Similarly, γ oil = 48:05 lbf=ft3 Calculate the velocity of oil by using the continuity equation (Eq. 3.8). The cross section area of flow is the area of the annual region. v=

m_ = ρA ρ π 4 250

= 48:05

lbm ft3

π 4

m_ Dio 2 - Doi 2 lbm sec

ð0:9950 ftÞ2 - ð0:5525 ftÞ2 = 9:67 ft=s

Calculate the Reynolds number using Eq. 3.29.

DH v Re = = ν

ft sec 0:000011 ft2 = sec 14:4 cSt × 1 cSt ð0:4425 ftÞ 9:67

= 27,014 2:70 × 104 Calculate the relative roughness using Eq. 3.16. Use DH in place of D. r=

ε 0:00015 ft = = 0:00034 DH 0:4425 ft

Using r and Re as parameters, determine the Fanning friction factor using Fig. 3.6 as shown. From the chart f = 0.0063.

202

3

Fluid Mechanics and Momentum Transfer

Calculate the head loss by using Eq. 3.14 and use the hydraulic diameter, DH, in place of the diameter, D. ft 2fLv2 ð2Þð0:0063Þð15 ftÞ 9:67 sec = hf = gDH 32:2 secft 2 ð0:4425 ftÞ

2

= 1:24 ft

Convert the friction head loss to pressure drop using Eq. 3.13. ΔP = γhf =

3.7

ð48:05 lbf Þð1:24 ftÞ ft3 144in2 ft2

= 0:414 psi

Flow Through Pipe Networks

In the process industry, pipes conveying process fluids often have to branch out of the main line for several reasons. For example, it is often necessary to have a pipe network to bypass a process unit. It is important to analyze and perform calculations related to pipe networks [4–6]. Figure 3.7 shows two parallel pipes in a simple pipe network. Apply the law of conservation of mass (continuity equation) at junctions A and B. Flow into the junction = Flow out of the junction Q = Q1 þ Q2

ð3:31Þ

However, the pressures and the heads at points A and B remain constant regardless of the path taken to reach point B from point A. Therefore, the pressure drop and head loss across Pipe 1 will be the same as the pressure drop and head loss across Pipe 2. The total flow distributes between pipes 1 and 2 in such a way that the pressure drops, and head losses are equal [4, 5]. Use the Fanning equation (Eq. 3.14) for calculating head loss. hL1 = hL2 2f 1 L1 v1 2 2f 2 L2 v2 2 = gD1 gD2

Fig. 3.7 Pipes in parallel

ð3:32Þ

3.8

Head Loss Through Fittings and Valves – Minor Losses

203

Hence, in parallel pipe-flow networks both Eqs. 3.31 and 3.32 must be simultaneously satisfied. Example 3.10 Assuming identical Fanning friction factors, determine the length, L1, of Pipe 1 in the piping network shown in the figure.

Solution The head loss across pipes 1 and 2 must be equal. Simplify Eq. 3.32, substitute the known values, and solve for L1.

3.8

Head Loss Through Fittings and Valves – Minor Losses

In addition to straight pipe, piping systems typically consist of fittings (elbows, tees, reducers, flanges) and valves. The friction losses across fittings and valves must be considered since they can contribute significantly to the overall friction head loss for piping systems with tight layouts in confined spaces, which is usually the case in chemical processing facilities. The friction losses across fittings and valves are described as minor losses. However, the term “minor losses” needs to be used with caution because the losses across fittings and valves can be “minor” only if the piping system has significant lengths of straight pipe and relatively few fittings and valves. There are two methods to calculate the friction head loss across fittings and valves [4, 5]: velocity head method and equivalent length method, both of which are presented in the following sections.

204

3.8.1

3

Fluid Mechanics and Momentum Transfer

Velocity Head Method

The velocity head for fluid flow is calculated by using the equation, h=

v2 2g

ð3:33Þ

As per Eq. 3.14, the friction head loss in a segment of a straight pipe is directly proportional to the velocity head, v2/2g. Similarly, the friction head loss across a fitting or valve will also be proportional to the velocity head. The velocity head is multiplied by an appropriate loss coefficient, K, to get the friction head loss across the fitting/valve. The friction head loss across fittings/valves can be calculated by the following equation: hfitting = K fitting

v2 2g

ð3:34Þ

The value of the loss coefficient, K, depends on the nature of the fitting and it can be obtained from any standard reference in fluid mechanics [4, 5]. Typical values of K for fittings and valves are shown in Table 3.2. Table 3.2 Typical values of loss coefficient, K, for flanged valves and fittings

Valve/fitting Gate valve Globe valve Diaphragm valve Ball valve Butterfly valve Angle valve Swing check valve 90° Elbow 45° Elbow Return bend Tee

Status/type Fully open Half open Fully open Half open Fully open Half open 5° Closed 40° Closed 5° Closed 40°Closed Open Open Standard (R = D) Long radius (R = 1.5D) Standard (R = D) Long radius (R = 1.5D) Flanged Line flow Branch flow

K 0.17 4.5 10 36 2.5 4.5 0.05 17.5 0.25 11 2 2 0.3 0.2 0.4 0.2 0.2 0.2 1.0

Note: The values provided in the preceding table must be used with caution and it is best to obtain the data from the vendors of the fittings

3.8

Head Loss Through Fittings and Valves – Minor Losses

205

Combining Eq. 3.34 with Eq. 3.14 for friction loss for a straight pipe, an equation for calculating the total friction head loss for the piping system can be obtained. htotal =

3.8.2

v2 2g

4f

L þ D

K

ð3:35Þ

Equivalent Length Method

In the equivalent length method of determining the minor losses, each fitting is represented by an equivalent length of straight pipe represented by Le [4, 5]. In terms of the equivalent length, the head loss for fittings can be calculated by substituting Le for the length in the Fanning equation (Eq. 3.14). hf = 4f

v2 2g

Le D

ð3:36Þ

The relationship between the loss coefficient and the equivalent length for fittings can be obtained by combining Eqs. 3.34 and 3.36. h=K

v2 v2 = 4f 2g 2g

Therefore, Le =

Le D

KD 4f

ð3:37Þ

Equation 3.37 provides a method for calculating equivalent length for fittings and valves. Table 3.3 lists typical values of the ratio of equivalent length to inside diameter (Le/D). Table 3.3 Equivalent lengths of valves and fittings

Valve/Fitting Gate valve Globe valve Butterfly valve Angle valve Swing check valve 90° Elbow 45° Elbow Return bend Tee

Status/Type Fully open Half open Fully open Open Open Open Standard (R = D) Long radius (R = 1.5D) Standard (R = D) Flanged Line flow Branch flow

(Le/D) 13 260 340 40 150 135 30 20 16 50 20 60

Note: The values provided in the preceding table must be used with caution and it is best to obtain the data from the vendors of the fittings

206

3

Fluid Mechanics and Momentum Transfer

The total length of the piping system including the equivalent lengths of fittings will be: Ltotal = Lpipe þ

Le,fittings

ð3:38Þ

The total head loss for a piping system including fittings can be calculated by using Ltotal in the Fanning equation (Eq. 3.14) for friction head loss: htotal = 4f

3.8.3

Ltotal D

v2 2g

ð3:39Þ

Other, Miscellaneous Minor Losses – Pipe Entrance, Pipe Exit, Change in Pipe Cross Section

Minor losses due to entrance of the fluid into a pipe, due to exit of fluid from a pipe, and due to changes in pipe cross section are calculated by using the loss coefficient method and the corresponding formula (Eq. 3.34). The loss coefficient, K, for different configurations of miscellaneous minor losses are summarized in the following table (Table 3.4). Example 3.11 A piping system consists of 50 ft of 10 in nominal (ID = 10.02 in, ε = 0.00015 ft) steel pipe. In addition, the piping system consists of the following fittings (all flanged): Fittings 90° LR elbows Fully open gate valves Fully open globe valve Fully open check valve

Qty 6 2 1 1

Le/D 20 20 350 150

2500 gpm water at 70 °F (ρ = 62.3 lbm/ft3, ν = 1.059 × 10-5 ft2/sec) flows from a source tank through the piping system into a process unit and all entities are in the same horizontal level. Calculate the total pressure drop across the piping system in psi using A. the velocity head method, using the loss coefficients from Table 3.2. B. the equivalent length method. Solution Determine the friction factor by calculating the Reynolds number. Convert the inside diameter to feet.

3.8

Head Loss Through Fittings and Valves – Minor Losses

207

Table 3.4 K values for miscellaneous minor losses Situation Pipe entrance

Illustration

Pipe exit

Sudden expansion

K 0.5

Remarks Loss due to fluid entering a pipe from a source tank

1.0

Loss due to fluid exiting a pipe into a destination tank

1-

D1 D2

Abrupt flow from smaller to larger pipe

2 2

Sudden contraction

0–0.5 depending on D2/D1 ratio

Usually taken as 0.5 for conservative estimation

Gradual expansion

Depends on D2/D1 ratio and θ

Usually taken as 0.3 for conservative estimation

Gradual contraction

Depends on D1/D2 ratio and θ

Usually taken as 0.05 for conservative estimation

D=

10:02 in = 0:835 ft 12 in ft

Calculate the velocity of water using the continuity equation (Eq. 3.9).

v=

Q = A

2500 gpm 449 gpm= ft3 = sec π 4 ð0:835

ftÞ2

Calculate the Reynolds number using Eq. 3.12.

= 10:17 ft= sec

208

3

Re =

Fluid Mechanics and Momentum Transfer

ft Dv ð0:835 ftÞ 10:17 sec = = 8:02 × 105 ft2 ν 1:059 × 10 - 5 sec

Calculate the relative roughness using Eq. 3.17. r=

ε 0:00015 ft = = 0:00018 ≃ 0:0002 D 0:835 ft

Using Reynolds number and relative roughness as parameters, determine the friction factor by using Fig. 3.6 as shown in the excerpt here. From the diagram, f = 0.0038.

A. First, calculate the velocity head using Eq. 3.33 and the ratio (L/D) 2

hvel =

ft 10:17 sec v2 = 1:606 ft = 2g 2 32:2 secft 2

L 50 ft = = 59:88 D 0:835 ft Calculate the total head loss using Eq. 3.35. Use the loss coefficients provided in Table 3.1 and include the entrance and exit losses. htotal =

v2 2g

4f

L þ D

=

v2 2g

4f

L þ 6ðK EL Þ þ 2ðK GTV Þ þ K GLV þ K CHKV þ K ent þ K exit D

K

= ð1:606 ftÞ½4ð0:0038Þð59:88Þ þ 6ð0:2Þ þ 2ð0:17Þ þ 10 þ 2 þ 0:5 þ 1 = 25:62 ft Convert the head loss in feet of water to psi using the appropriate conversion factor.

3.9

The Mechanical Energy Equation

209

ΔPpsi = ð25:62 ft of waterÞ

1 psi = 11:09 psi 2:31 ft of water

B. Calculate the sum of the equivalent lengths of the fittings by using the given data. Le = ð0:835 ftÞð6 × 20 þ 2 × 20 þ 1 × 350 þ 1 × 150Þ D

Le = D = 551 ft

Calculate the total equivalent length of the piping system using Eq. 3.38. Ltotal = Lpipe þ

Le,fittings = 50 ft þ 551 ft = 601 ft

Calculate the head loss across the piping system using Eq. 3.39 and add the entrance and exit losses. htotal = 4f =

Ltotal D

v2 2g

4f

v2 v2 þ ðK ent þ K exit Þ 2g 2g Ltotal D

þ K ent þ K exit

= ð1:606 ftÞ ð4 × 0:0038Þ

601 ft þ 0:5 þ 1 0:835 ft

= 19:98 ft of water Convert the head loss in feet of water to psi using the appropriate conversion factor. ΔPpsi = ð19:98 ft of waterÞ

1 psi = 8:65 psi 2:31 ft of water

Note: The discrepancy in the results from the two methods is due to wide variation in the data available for loss coefficients and equivalent lengths. In practice, it is best to obtain this data from the vendors.

3.9

The Mechanical Energy Equation

The different forms of energy associated with a flowing fluid are pressure energy, potential energy, kinetic energy, and internal energy. The internal energy of a substance changes only if there is a change in the temperature of the substance.

210

3

Fluid Mechanics and Momentum Transfer

Table 3.5 Energy forms and equations Energy form Pressure energy

Equation for total energy PV

Potential energy

mgz

Kinetic energy

mv2/2

Equation for energy per unit weight Pr:Energy PV PV P = = = Unit wt: W γV γ Potential Energy mgz mgz = = =z Unit wt: W mg 2 2 mv v2 Kinetic Energy mv = = = Unit Wt: 2W 2mg 2g

Fig. 3.8 Control volume for energy equation

Since the temperature of a flowing fluid typically remains constant, the internal energy of the fluid remains constant. Equations for each form of energy and the corresponding equations for energy per unit weight of the fluid are given here. Note that the weight of the fluid is the specific weight times the volume of the fluid, that is, W = γ V. The weight of the fluid is also the mass of the fluid times the acceleration due to gravity, that is, W = m g. Table 3.5 summarizes the expressions for different forms of energy including equations for total energy and equations for energy per unit weight. Consider the control volume of the fluid between points 1 and 2 in the pipe shown in the figure (Fig. 3.8). Application of the first law of thermodynamics for the control volume (CV) results in the following set of equations. Total energy of the fluid at point 1 þEnergy added to the control volume - Energy removed from the control volume = Total energy of the fluid at point 2

ð3:40Þ

Energy can be added to the fluid in the form of pressure head added by a pump between points 1 and 2. The pressure head added by a pump is typically represented by hPump with units of feet (ft) or meter (m). As the fluid moves from point 1 to point 2, it has to overcome the friction at the pipe surface, which results in a loss of energy of the fluid [3–6]. This loss in energy is termed as the friction head loss, represented as hf (in feet or meter). The friction head loss for flow through a pipe can be calculated using the Fanning equation (Eq. 3.14). The energy terms in Eq. 3.40 can be written in terms of energy per unit weight (as shown earlier) resulting in the following equation.

3.9

The Mechanical Energy Equation

211

v2 P1 v21 P þ þ z1 þ hPump - hf = 2 þ 2 þ z2 γ 2g γ 2g

ð3:41Þ

Equation 3.41 is the mechanical energy equation [4, 5], also know n as the “extended Bernoulli’s equation” [4] for fluid flow between points 1 and 2 in a flow field. The kinetic energy per unit weight, which is v2/2g, is also known as “velocity head.” Each term in the mechanical energy equation has the units of head of the liquid (in feet or meter). The energy lines can be plotted with respect to an arbitrary datum line. The sum of pressure head and elevation is known as “hydraulic grade line (HGL)” and when the velocity head is added to the hydraulic grade line, the “energy grade line (EGL) is obtained [4]. The mechanical energy equation can also be written in terms of energy per unit mass with each term in the equation having the units of ft-lbf/lbm (USCS) or N.m/kg (SI). The energy terms per unit mass in USCS and SI units are: lbf 2 Pressure Energy PV P P ft - lbf = = m =  ft  ðUSCSÞ lbm Unit mass m ρ lbm V ft3 N Pressure Energy PV P P m2 N:m J  = =m=   ðS IÞ kg Unit mass m ρ kg kg V m3 Potential Energy mgz = = gz  Unit mass m 

lbm - ft sec 2

Potential Energy mgz = = gz  Unit mass m m  2 ðmÞ × s Therefore,

ft ðftÞ  sec 2 ft lbm

 ðlbf Þ

m ðm Þ s2 kg kg  m  kg s2

ft lbm ðftÞ × lbm sec 2 ft lbm

m kg



ft - lbf ðUSCSÞ lbm



Nm J  ðS IÞ kg kg

ft2 ft‐lbf m2 J  and 2  2 lbm kg sec s ft2 Kinetic Energy mv2 v2 ft‐lbf  = =  ðUSCSÞ 2m 2 Unit mass lbm sec 2 J Kinetic Energy mv2 v2 m2 ðS IÞ =  2  = kg 2m 2 Unit mass s

The units for the energy added by the pump per unit mass of the fluid are determined as shown here.

212

3

mghPump  ghPump  m

Fluid Mechanics and Momentum Transfer

ft ft‐lbf ft2  ð ft Þ  ðUSCSÞ lbm sec 2 sec 2

mghPump m J m2 ðS IÞ  ghPump  2 ðmÞ  2  kg m s s The energy terms in Eq. 3.40 can be written in terms of energy per unit mass using the preceding results. v2 P1 v21 P þ þ z1 g þ hPump g - hf g = 2 þ 2 þ z2 g ρ 2 ρ 2

ð3:42Þ

Example 3.12 Fuel oil at 60 °F is pumped to an open tank by using a 12 in, Sch. 40 steel pipe [ID = 11.94 in]. The equivalent length of the piping system is 4000 ft. The flow rate of the oil is 3000 gpm. The oil surface in the tank is 300 ft above the pump suction line. The specific gravity of the oil is 0.86 and its kinematic viscosity is 5.547 × 105 2 ft /sec. The suction pressure at the pump inlet is 2 psig. Determine the head added by the pump in feet and the discharge pressure of the pump. Solution Determine the friction head loss in the pipe by using the following steps. Convert the pipe diameter to feet. D=

11:94 in = 0:995 ft 12 in ft

Calculate the velocity in the pipe by using the continuity equation (Eq. 3.9).

v=

Q Q = = A πD2 4

ð3000 gpmÞ

1 ft3 = sec 449 gpm

π 4 ð0:995

ftÞ2

= 8:593 ft= sec

Calculate the Reynolds number for the flow by using Eq. 3.12. Re =

ft Dv ð0:995 ftÞ 8:593 s = 1:54 × 105 = 2 ν 5:547 × 10 - 5 fts

The roughness of a commercial steel pipe is ε = 0.00015 ft. Using Eq. 3.16 the relative roughness is,

3.9

The Mechanical Energy Equation

r=

213

ε 0:00015 ft = = 0:00015 D 0:995 ft

Locate the intersection of the ordinate at Re = 1.54 × 105 and the curve for r = 0.00015, as shown in the accompanying figure. At the point of intersection, f = 0:0045 Calculate the friction head loss in the piping system using Eq. 3.14.

hf = 4f

L D

4000 ft v2 = 4ð0:0045Þ 2g 0:995 ft

8:593 ð2Þ 32:2

ft 2 s

ft s2

= 83 ft of oil Obtain an expression for the head added by the pump, hPump, from the mechanical energy equation (Eq. 3.41). hPump =

P2 - P1 v22 - v21 þ þ ðz2- z1 Þ þ hf γ 2g

Take the reference point 1 as the suction of the pump and reference point 2 as the oil surface in the open tank. Calculate the specific weight of the oil by using the definition of specific gravity, (Eq. 3.1). γ oil = ðSGoil Þ γ water, std = ð0:86Þ 62:4

lbf ft3

= 53:66 lbf=ft3 Since the oil surface is open to the atmosphere, P2 (gage) = 0 psig. The pressure at the pump suction is P1 = 2 psig. Since the cross section of the tank is large and since the level of oil in the tank is constant, the velocity of the oil in the tank is zero, that is, v2 = 0.

214

3

Fluid Mechanics and Momentum Transfer

The velocity of oil on the suction side of the pump is v1 = 8.593 ft/sec. The oil surface is 300 ft above the suction pipe, therefore, z2 - z1 = 300 ft. The friction head loss in the pipe is hf = 83 ft. Substitute all the known values into the equation obtained for the pump head. P2 - P1 v22 - v21 þ þ ð z 2 - z 1 Þ þ hf γ 2g 144 in2 lbf 0- 2 2 ft 2 0 - 8:593 sec ft2 in = þ ft lbf 53:66 3 2 × 32:2 sec 2 ft þ300 ft þ 83 ft

hPump =

= 377 ft Convert the head added by the pump to equivalent pressure difference across the pump by using Eq. 3.4. ΔPPump,psi = γ oil hPump = 53:66

lbf 1 ft2 ð377 ftÞ = 140:5 psi 3 144 in2 ft

Calculate the discharge pressure of the pump. Pdisch: = Psuction þ ΔPpump = 2 psi þ 140:5 psi = 142:5 psi

3.9.1

Bernoulli’s Equation

For frictionless flow between points 1 and 2, and in the absence of turbomachinery such as pumps, hf and hPump can be set to zero in the mechanical energy equation (Eq. 3.41). This simplification results in the Bernoulli’s equation (Eq. 3.43): v2 P1 v21 P þ þ z1 = 2 þ 2 þ z 2 γ 2g γ 2g

ð3:43Þ

Bernoulli’s equation [4–6] states that the sum of the pressure head, the velocity head, and the potential head (elevation) is constant in a frictionless flow field without any energy-producing or energy-consuming devices. Bernoulli’s equation has important applications in deriving equations for flow meters such as orifice meters and venturi meters discussed later in this chapter.

3.9

The Mechanical Energy Equation

3.9.2

215

Pump Power Equation

Pumps add energy to the flowing fluid. The rate of energy transferred to the fluid by the pump, expressed in horsepower (hp), is the “hydraulic horsepower (HHP)” of the pump. Hydraulic horsepower is also referred to as “water horsepower” [3–5]. As noted earlier, the head added by the pump represents the energy added per unit weight of the fluid. Since power is the rate of consumption of energy, the hydraulic power consumed by the fluid is the weight flow rate of the fluid times the head supplied by the pump. The weight flow rate of the fluid is the volume flow rate multiplied by the specific weight of the fluid. Recall that the specific weight of a fluid is the weight per unit volume. Also, horsepower is the consumption of energy at the rate of 550 ft-lbf/sec. If the volume flow rate, Q, is in ft3/sec and the specific weight, γ, is in lbf/ft3, and the head added by the pump, hPump, is in ft, then the hydraulic horsepower can be calculated by Eq. 3.44. HHP =

Qcfs γhPump 550 ft‐lbf= sec hp

ð3:44Þ

Typically, the volume flow rate is in gallons per minute (gpm) and 1 ft3/ sec = 449 gpm. The specific weight of the fluid is the specific gravity (SG) of the fluid multiplied by standard specific weight of water, 62.4 lbf/ft3. Making these substitutions into Eq. 3.44 results in the following equation for calculating the hydraulic horsepower for any fluid, with, the volume flow rate, Q, in gpm [5]. HHP =

Qgpm ðSGÞhPump 3960 ft‐gpm hp

ð3:45Þ

Since pumps are not 100% efficient, the power supplied to the pump should be more than the required hydraulic horsepower. The actual mechanical horsepower supplied to the pump is called the “brake horsepower (BHP).” Therefore, BHP =

Qgpm ðSGÞhPump HHP = ft‐gpm ηPump η Pump 3960 hp

ð3:46Þ

Pumps are driven by electric motors, which operate at less than 100% efficiency. Therefore, the electrical power to be supplied to the pump must factor in the efficiency of the motor. The electrical power is typically calculated in kilowatts (kW) and therefore the energy consumption will be in kilowatt hours (kWH). Since 1 hp = 0.746 kW, the electrical power consumption can be calculated by using Eq. 3.47.

216

3

PðkWÞ = 0:746

kW hp

Fluid Mechanics and Momentum Transfer

HHP ηPump  ηMotor

ð3:47Þ

The pump power equations in SI Units [5] are given here. Hydraulic power ðkWÞ = QγhPump

ð3:48Þ

In Eq. 3.48, Q is in m3/s, γ is in kN/m3, and hPump is in m. Explanation of units in Eq. 3.48: Hydraulic power = QγhPump 

m3 s

kN  m kJ kN ðm Þ    kW s s m3

Brake power ðkWÞ = Electrical power ðkWÞ =

QγhPump ηPump

ð3:49Þ

QγhPump ηPump  ηMotor

ð3:50Þ

Example 3.13 A centrifugal pump takes in water from an open tank and discharges it to a process unit operating at 35 bar(g). The flow rate of water is 71 L/s and the discharge pipe at the process unit is 25 m above the water level in the supply tank. The brake power of the pump is measured to be 345 kW. Velocity heads are typically small relative to other terms, and they can be ignored. In this case, the friction head loss in the piping system is 3.5 m. Determine A. the head supplied by the pump in meters (m) B. the efficiency of the pump Solution A. Solve the mechanical energy equation (Eq. 3.41) for the head added by the pump, hPump. hPump =

P2 - P1 v22 - v21 þ þ ðz2- z1 Þ þ hf γ 2g

Apply the mechanical energy equation between the water surface in the source tank (reference point 1) and the discharge point in the process unit (reference point 2). Since the water surface in the supply tank is open to the atmosphere, P1 (gage) = 0 kPa.

3.9

The Mechanical Energy Equation

217

The pressure at the discharge point in the process unit is P2 = ð35 barÞ

100 kPa = 3500 kPa 1 bar

The specific weight of water is γ = 9.81 kN/m3. Since the cross section of the supply tank is large and since the level of water in the tank is constant, the velocity of the water in the supply tank is zero, that is, v1 = 0 and the velocity head

v22 2g

can also be ignored.

The discharge pipe at the process unit is 25 m above the water level in the supply tank, therefore, z2 - z1 = 25 m. The friction head loss in the pipe is hf = 3.5 ft. Substitute all the known values into the equation developed for the pump head. hPump =

P2 - P1 v22 - v21 þ þ ðz2 - z1 Þ þ h f γ 2g

3500 kPa - 0 kPa þ 0 þ 25 m þ 3:5 m kN 9:81 3 m = 385:3 m

=

2

kN=m kPa Note: kN=m 3  kN=m3 = m

B. Solve Eq. 3.49 for the efficiency of the pump. ηPump =

QγhPump Brake Power ðkWÞ

L 1 m3 kN 71 × 9:81 3 ð385:3 mÞ s 1000 L m = 345 kW = 0:7778ð ≃ 78%Þ kJ Note: kNm s  s  kW

Example 3.14 A hydraulic power recovery unit uses high pressure water from a process unit. The water enters the hydraulic turbine at 400 psig at a flow rate of 1800 gpm and discharges from the turbine at 20 psig at a location 20 ft below the turbine inlet. Determine kW of power produced by the power recovery unit if the efficiency of the turbine is 75%.

218

3

Fluid Mechanics and Momentum Transfer

Solution Draw the schematic diagram for the hydraulic turbine as shown.

Apply the mechanical energy equation (Eq. 3.41) between points 1 and 2. In this case, hturbine is the head developed by the turbine on the downstream side. Assume no friction losses within the turbine and that the velocity heads are small and cancel out.

P1 - P2 þ ðz1 - z2 Þ γ lbf 144 in2 lbf 144 in2 - 20 2 × 400 2 × 2 ft ft2 þ 20 ft in in = lbf 62:4 3 ft = 897 ft

hturbine =

Calculate the horsepower developed by the turbine by using Eq. 3.46 and then convert the horsepower to kW using the appropriate conversion factor. In this case the power produced will be less than the ideal power because of the losses due to the efficiency of the turbine. Hence, the hydraulic power will have to be multiplied by the efficiency of the turbine to get the actual power.

HPturbine =

Qgpm ðSGÞhturbine ð0:75Þ ft‐gpm 3960 hp

ð1800 gpmÞð1:0Þð897 ftÞ ð0:75Þ ft‐gpm 3960 hp = 306 hp =

kWturbine = 306 hp × 0:746

kW = 228 kW hp

3.9

The Mechanical Energy Equation

219

Fig. 3.9 Pump performance curves

3.9.3

Pump Performance Parameters

Several parameters are used in evaluating and representing the performance of pumps. The parameters are obtained by testing the pump and they are plotted to generate the “pump performance curves” [4, 5]. While plotting the performance curves of a centrifugal pump, the volume flow rate, Q (gpm or m3/s), is used as the independent variable. The typical parameters plotted to obtain the centrifugal pump performance curves are: 1. Head (H ) vs. capacity (Q) for different impeller diameters. Note: The head supplied by a pump is also known as “total dynamic head (TDH)” [4, 5]. 2. Power required to drive the pump (BHP) vs. capacity (Q). 3. Pump efficiency (η) vs. capacity (Q). 4. Net positive suction head required, (NPSHR) vs. capacity (Q). Figure 3.9 illustrates typical performance curves for a centrifugal pump. Example 3.15 A centrifugal pump with characteristics shown in the figure below and an impeller diameter of 241 mm is used in pumping 3.20 L/s of a liquid with SG = 1.15. Determine A. the total dynamic head supplied. B. the kW brake power required.

220

3

Fluid Mechanics and Momentum Transfer

Solution

A. The performance curve plots the dynamic head supplied vs. flow rate. The specific gravity of the liquid will not affect the volume flow rate. Hence, the dynamic head supplied will be the same as that indicated by the performance curve for water. Locate the intersection of the 3.20 L/s volume flow rate ordinate with the performance curve for the 241 mm diameter impeller. From the intersection point, read the dynamic head supplied on the y-axis as shown in the figure. In this case, the total dynamic head supplied is 25 m. B. As shown in the graph, the efficiency at the point of interest is 59% by interpolation between the given efficiency curves. Calculate the brake kW required using Eq. 3.49.

3.10

Flow Measurement

221

Brake Power =

QγhPump ηPump 3:2

=

L 1 m3 × s 1000 L

= 1:53 kW

1:15 × 9:81

kN ð25 mÞ m3

0:59

Note: The specific weight of the liquid is 1.15 × sp.wt. water, and the standard sp.wt. of water is 9.81 kN/m3. kJ Units: kNm s  s  kW

3.10 3.10.1

Flow Measurement Introduction

In fluid flow systems, it is necessary to measure and monitor fluid flow parameters such as velocity of flow, volume flow rate, and pressure drop [3, 5, 6]. These parameters are measured by using suitable devices based on the applications of Bernoulli’s equation (Eq. 3.43). Pitot tubes are used in measuring point velocities in a flow field. Orifice meters and venturi meters are used in measuring volume flow rates. Pressure drops in piping systems are measured using differential manometers and differential pressure gages, discussed earlier in this chapter.

3.10.2

Pitot Tube

A pitot tube is used in measuring fluid velocities in ducts and pipes [3, 5, 6]. As shown in Fig. 3.10, it consists of a bent stagnation tube that is inserted into the flow system.

Fig. 3.10 Pitot tube with differential manometer

222

3

Fluid Mechanics and Momentum Transfer

The velocity at the stagnation point is zero. This results in a higher pressure, P0, at the stagnation point. The pressure downstream of the stagnation point is the static pressure, Ps. The velocity of the fluid is proportional to the pressure difference between the stagnation pressure and the static pressure. ΔP = P0 - Ps

ð3:51Þ

The pressure difference can be measured by a differential manometer or by a differential pressure gage. In Fig. 3.10, a differential manometer shows a reading of hm. The equation for the velocity of the fluid can be obtained by applying Bernoulli’s equation (Eq. 3.43) between the stagnation point and the downstream static point, assuming negligible friction loss between the two points.

v=

2gð

P0 - Ps Þ= γ

2gð

ΔP Þ γ

ð3:52Þ

In Eq. 3.52, γ = specific weight of the flowing fluid = γ f Example 3.16 The velocity of a process gas in circular duct is determined to be 80 ft/sec. The density of the gas is 0.088 lbm/ft3. Determine the reading of a differential water manometer connected across the pitot tube in inches of water. Solution The flowing fluid is a process gas. Calculate the specific weight of the gas by using the equation for specific weight (USCS units). γf =

ft Þð32:2 sec ρf g ð0:088 lbm 2Þ ft3 = = 0:088lbf=ft3 2 lbm - ft=sec gc 32:2 lbf

Calculate the difference between the stagnation and static pressure by using Eq. 3.52.

v γf = ΔP = 2g 2

80

ft sec

2

0:088

ft ð2Þ 32:2 sec 2

lbf ft3

= 8:745 lbf=ft2

Using the standard specific weight of water (62.4 lbf/ft3) and using Eq. 3.5, calculate the reading of the differential water manometer.

3.10

Flow Measurement

hm =

ΔP = γm - γf

223

8:745lbf ft2 = 0:1403 ft ð 1:68 in waterÞ lbf lbf 62:4 3 - 0:088 3 ft ft

3.10.3 Orifice Meter An orifice meter [3, 5, 6] is used in measuring the flowrate of a fluid through a pipe. It consists of an orifice plate with a circular opening that is concentric with the pipe. The diameter of the orifice, Do, is usually half the diameter of the pipe. The streamlines shown in Fig. 3.11 indicate that the smallest flow area is downstream of the orifice plate. The section with the smallest flow area (section 2 in Fig. 3.11) is known as “vena contracta.” Pressure taps are provided far upstream (section 1) of the orifice plate and downstream at the vena contracta as shown in Fig. 3.11. Due to the reduction in area, the velocity increases at the vena contracta resulting in a decrease of pressure at the vena contracta. The velocity and hence the flow rate through the pipe is proportional to the pressure difference between section 1 and section 2 shown in Fig. 3.11. The pressure difference can be measured by a differential manometer or by a differential pressure gage [5]. To obtain the equation to calculate the volume flow rate, apply the Bernoulli’s equation (Eq. 3.43) between the upstream point (1) and the vena contracta (2).

ð3:53Þ Apply the continuity equation (Eq. 3.9), to sections “1,” “2,” and “o.” Q = A1 v1 = A2 v2 = Ao vo

ð3:54Þ

The ratio of the area at vena contracta to the orifice area is known as the coefficient of contraction, Cc. The coefficient of contraction, Cc, is used in Fig. 3.11 Orifice meter with differential manometer

224

3

Fluid Mechanics and Momentum Transfer

expressing the velocity at the vena contracta in terms of the velocity at the orifice. Using Eq. 3.54, vo = v2

A2 = C c v2 Ao

ð3:55Þ

The equation for the volume flow rate of the fluid can be obtained by combining Eqs. 3.53, 3.54, and 3.55. Q=

C c Ao 1 - C2c AAo1

2

2g

P1 - P2 = γ

C c Ao 1 - C 2c AAo1

2

2gΔP γ

ð3:56Þ

For greater accuracy, a correction factor known as coefficient of velocity, Cv, is applied to Eq. 3.56. Q=

Cv Cc Ao 1 - C2c AAo1

2

2g

P1 - P2 = γ

C v C c Ao 1 - C 2c AAo1

2

2gΔP γ

ð3:57Þ

Equation 3.57 is commonly presented in a simplified form as shown in Eq. 3.58, which is used in practice to calculate the volume flow rate. Q = Co Ao

2g

P1 - P2 = C o Ao γ

2gΔP γ

ð3:58Þ

In Eq. 3.58, Co is known as the orifice coefficient (or the discharge coefficient) and it can be calculated by using the following equation. Co =

Cv Cc 1 - C2c

Ao A1

2

For a sharp-edged orifice, the most commonly used type of orifice, the orifice coefficient is usually taken as Co = 0.61. Example 3.17 A differential mercury manometer connected across an orifice meter shows a reading of 6 cm of mercury (SGm = 13.6). The pipe diameter is 254 mm, and the orifice diameter is 127 mm. Calculate the flow rate of water through the pipe in liters per second.

3.10

Flow Measurement

225

Solution Convert the manometer reading into an equivalent differential pressure by using Eq. 3.6. ΔP = P1 - P2 = γ f hm ðSGm - 1Þ =

9:81

kN m3

6 cm ð13:6 - 1Þ 100 cm 1m

= 7:42 kN=m2 ðkPaÞ Calculate the cross section area of the orifice. 2

ðπ Þ Ao =

πDo 2 = 4

127 mm 1000 mm 1m 4

= 0:01267 m2

Calculate the volume flow rate by using Eq. 3.58. Q = C o Ao

2g

ΔP γw m ð2Þ 9:81 2 s

= ð0:61Þð0:01267 m2 Þ

kN 7:42 2 m kN 9:81 m3

= 0:02977 m3 =s Convert the volume flow rate to liters per second. Q=

0:02977

m3 s

1000 L m3

= 29:77 L=s

3.10.4

Venturi Meter

A venturi meter [3, 5, 6] consists of a narrow section (called the “throat”) that is concentric with the pipe. Pressure taps are provided upstream of the venture meter and at the throat as shown in Fig. 3.12.

226

3

Fluid Mechanics and Momentum Transfer

Fig. 3.12 Venturi meter with a differential manometer

The velocity of the fluid is higher at the throat due to the constriction. As per Bernoulli’s equation, higher velocity results in a lower pressure at the throat. The velocity and hence the flow rate through the pipe is proportional to the pressure difference between the pipe section and the throat section. Applying the Bernoulli’s equation (Eq. 3.43) between upstream section (1) and the throat section (2) and simplification results in the following equations.

ð3:59Þ Apply the continuity equation (Eq. 3.9) at sections “1” and “2.” Q = A1 v1 = A2 v2 Therefore, v1 =

ð3:60Þ

A2 v A1 2

ð3:61Þ

Combining Eqs. 3.59, 3.60, and 3.61 and applying the velocity correction factor (coefficient of velocity, Cv), results in the following equation for the volume flow rate of the fluid. Q=

C v A2 1-

A2 A1

2

2g

P1 P2 = γ γ

Cv A2 1-

A2 A1

2

2g

ΔP γ

ð3:62Þ

The ratio of the throat diameter to the pipe diameter is represented by β. Hence, β=

D2 D1

3.10

Flow Measurement

227

A2 A1

2

ðπ=4ÞD2 2 ðπ=4ÞD1 2

=

2

=

D2 D1

4

= β4

ð3:63Þ

Combining Eqs. 3.62 and 3.63 results in Eq. 3.64, which is commonly used for calculating the volume flow rate in a venturi meter. Q=

Cv A2 1-β

4

2g

ΔP γ

ð3:64Þ

In Eq. 3.64, the velocity coefficient, Cv, is usually taken as 0.98. Example 3.18 A differential pressure gage connected across a venturi meter shows a reading of 0.90 psi. The pipe diameter is 12 in, and the throat diameter is 6 in. Determine the flow rate of water through the pipe in gpm. Solution Calculate the ratio of the throat diameter to the pipe diameter. β=

6 in D2 = = 0:50 D1 12 in

Calculate the cross section area of the throat. A2 =

2 πD2 2 π ð0:50 ftÞ = = 0:1963 ft2 4 4

Calculate the volume flow rate using Eq. 3.64. Q=

=

C v A2 1-β

4

2g

ΔP γ

ð0:98Þ 0:1963 ft p 1 - 0:54

2

2 32:2

ft sec 2

0:90

144 in2 ft2 lbf 62:4 3 ft

lbf in2

= 2:298 ft3 = sec ðcfsÞ Convert the volume flow rate to gallons per minute. Q = 2:298

ft3 sec

449 gpm = 1032 gpm ft3 = sec

228

3

Fluid Mechanics and Momentum Transfer

3.10.5 Orifice Meter and Venturi Meter Comparison The advantage of orifice meter over a venturi meter is its simple construction and ease of dismantling for servicing and maintenance purposes [3, 5, 6]. However, due to the sudden constriction at the orifice plate, the pressure recovery is extremely poor in an orifice meter resulting in a higher permanent pressure loss. In contrast, a venturi meter has gradually convergent and divergent sections into and out of the throat area resulting in a lower permanent pressure loss. The permanent pressure loss in an orifice meter depends on the ratio of the orifice diameter to the pipe diameter, represented by β. β=

Do D1

ð3:65Þ

Depending on the value of β, the permanent pressure loss in an orifice meter can range from 50% to 90% of the total pressure loss across the meter [3, 5, 6]. In a venturi meter, the permanent pressure loss is usually about 10% of the total pressure loss.

3.10.5.1

Calculation of Permanent Pressure Loss in an Orifice Meter

The permanent pressure loss in an orifice meter can be calculated by using the following formula: ΔPPerm = 1- β2 ΔPTotal

ð3:66Þ

Example 3.19 A differential pressure gage connected across an orifice meter shows a reading of 0.18 bar(g). The orifice diameter is 6 cm, and the internal diameter of the pipe is 15 cm. Determine the permanent pressure drop across the orifice meter. Solution Calculate the ratio of the orifice diameter to the pipe diameter using Eq. 3.65. β=

6 cm Do = = 0:4 D1 15 cm

Calculate the permanent pressure drop using Eq. 3.66. ΔPPerm = 1- β2 ΔPTotal = 1- 0:42 ð0:18 barÞ = 0:1512 bar

3.11

3.11

Flow Through Packed Beds

229

Flow Through Packed Beds

Many chemical processing operations and facilities use packed columns to accomplish unit operations such as gas absorption/desorption, and distillation. The packing in the columns serve to increase the surface area available for diffusion and mass transfer. The fluid dynamics of flow through packed columns involve calculation of the pressure drop across the packing and also the estimation and prediction of imminent flooding conditions within the column [2, 3].

3.11.1 Calculation of Pressure Drop in Packed Columns The resistance to flow of fluids in packed columns is primarily due to the drag force of the packing particles. Fluid flow in packed beds occurs through randomly shaped interconnected channels. However, the assumption of uniform circular channels in the bed simplifies the calculation basis and the subsequent equations can be modified by using experimental results. Based on the preceding approach, in addition to the density, viscosity, and velocity of the fluid, the following parameters affect the flow of fluids and hence the pressure drop in a packed column [2, 3]. 1. Porosity of the bed, ε, which is a measure of the void space available for fluid flow. ε=

void volume within the packing total volume of the packing

ð3:67Þ

2. Equivalent spherical diameter of the particle, which is influenced by the volume to surface area ratio of the particle.

DP = 6

V volume of the particle =6 P SP surface area of the particle

ð3:68Þ

The velocity of the fluid will be higher compared to the velocity in the empty tower due to the constrictions created by the packing. The superficial velocity, Vs, also known as the velocity based on empty tower cross section, is defined as the volume flow rate of the fluid divided by the cross section of the tower. vS =

Q Acs

ð3:69Þ

230

3

Fluid Mechanics and Momentum Transfer

The Ergun equation, commonly used in calculating the pressure drop across a packed bed of length, L, is 2 1:75ρv2S 1 - ε ΔP 150vS μð1 - εÞ þ = L DP ε3 D2P ε3

ð3:70Þ

The friction factor and the Reynolds number for flow through packed beds are defined by the following equations. fP =

ε3 1-ε

ΔP L

DP ρv2S

Re P =

DP vS ρ ð1 - εÞμ

ð3:71Þ ð3:72Þ

Based on the preceding definitions, the Ergun equation can be written in terms of the friction factor and Reynolds number as follows. fP =

150 þ 1:75 Re P

ð3:73Þ

Example 3.20 Water flows through a packed bed of height 6 ft and porosity of 0.28. The bed is supported in a tower with diameter 3 ft. The packing consists of 2 in intalox saddles with surface area to volume ratio of 35 ft2/ft3. The properties of water at the flow temperature are density, ρ = 62.4 lbm/ft3, and dynamic viscosity, μ = 6.72 × 104 lbm/ft ‐ sec. The velocity of water through the bed should be such that the pressure drop across unit height of the bed is constrained to be ΔP/L = 1.34 psi/ft. Determine, A. the maximum possible flow rate of water (gpm). B. the Reynolds number for water flow. C. the friction factor for the flow of water. Solution Calculate the equivalent spherical diameter of the particle using Eq. 3.68. DP = 6

1 ft3 VP =6 = 0:1714 ft 35 ft2 SP

Substitute all the known values into Ergun equation (Eq. 3.70) and solve for the superficial velocity.

3.11

Flow Through Packed Beds

231

Note that the Ergun equation in USCS units requires the use of the conversion constant gc to ensure consistency of units. 2 1:75ρv2S 1 - ε ΔP 150vS μð1 - εÞ þ = DP gc L ε3 D2P ε3 gc

)

1:34

lbf 144 in2 ð150Þðv Þ 6:72 × 10 - 4 lbm ð1 - 0:28Þ2 S × ft‐sec in2 ft2 = ft lbm‐ft=sec2 ð0:1714 ftÞ2 ð0:28Þ3 32:2 lbf

lbm ð vS Þ 2 ft3 þ lbm‐ft=sec2 ð0:1714 ftÞ 32:2 lbf ð1:75Þ 62:4

lbf 1 - 0:28 ) 192:96 3 0:283 ft

= 2:516ðvS Þ þ 648:95ðvS Þ2 ) 3:363v2s þ 0:013vS - 1 = 0 Solve the preceding quadratic equation for vS. vS = 0:5434 ft= sec: A. Calculate the volume flow rate of water using Eq. 3.69. vS =

π ð3 ftÞ2 = 3:841 ft3 = sec 4

Q ft ) Q = vS Acs = 0:5434 Acs sec

Convert the volume flow rate to gallons per minute (gpm). Q = 3:841

ft3 sec

449 gpm ft3 sec

= 1726 gpm

B. Calculate the Reynolds number using Eq. 3.72.

D vρ Re P = P S = ð1 - εÞμ

lbm ft3 lbm ð1 - 0:28Þ 6:72 × 10 - 4 ft‐ sec

ð0:1714 ftÞ 0:5434

ft sec

62:4

= 12012

232

3

Fluid Mechanics and Momentum Transfer

C. Calculate the friction factor using Eq. 3.73. fP =

150 150 þ 1:75 = þ 1:75 = 1:7625 Re P 12012

Note: The same result can be obtained using Eq. 3.71. However, the numerator of Eq. 3.71 must be multiplied by the conversion constant gc for consistency of units.

3.11.2

Operation of Packed Columns – Generalized Pressure Drop Correlations and Flooding

Hydraulic design of packed columns involves providing an appropriate diameter of the column to avoid excessive pressure drop and flooding [2, 3]. The data required for such a design is based on extensive experimental studies which have resulted in useful correlations involving liquid and gas flow rates, properties of the fluids involved, and the characteristics of the packing used.

3.11.2.1

Generalized Pressure Drop Correlations

Extensive experimental studies have been conducted on the operation of packed columns and the results have been presented as generalized pressure drop correlation (GPDC) graph. Figure 3.13 is a GPDC plot in USCS units and Fig. 3.14 is a GPDC plot in SI units (nomenclature provided after solution to Example 3.21). A GPDC graph is essentially a plot of the dimensionless gas flow rate vs. liquid to gas flow ratio with pressure drop across unit height of the bed as parametric curves. Example 3.21 illustrates the application of GPDC in a packed tower. The nomenclature applicable to Fig. 3.13 is as follows: L, G: liquid and gas mass velocities (lbm/ sec ‐ ft2) ρL, ρG: liquid and gas densities (lbm/ft3) μL: liquid viscosity (cP) g: acceleration due to gravity (32.2 ft/sec2) FP: packing factor (ft-1) Example 3.21 A volatile organic component is to be removed from air using water as the solvent. The gas flow rate is 0.3155 lbm/sec and the liquid to gas mass flow ratio is 1.35. The packing used is 1 in Berl Saddle. It has been determined that a pressure drop of 0.06 psi per foot of packing height will ensure operation of the column without flooding. The relevant fluid properties and packing characteristics are:

3.11

Flow Through Packed Beds

233

Generalized Pressure Drop Correlation (GPDC – USCS Units)

Fig. 3.13 Generalized pressure drop correlation – USCS units. (Generated by the author, N.S. Nandagopal, P E)

ρL = 62:4 lbm=ft3 , μL = 1:05 cP, ρG = 0:0945 lbm=ft3 Packing factor, FP = 110 ft-1. Determine the tower diameter required for floodfree operation. Solution Determine the value of the parameter on the x-axis of the GPDC graph (Fig. 3.13). L G

ρG = ð1:35Þ ρL - ρG

0:0945 lbm m3 = 0:0526 lbm 62:4 lbm 0:0945 3 m m3

Using the preceding value of the x-axis parameter and the specified pressure drop parametric curve, determine the value of the parameter on the y-axis of the GPDC graph as shown.

234

3

Fluid Mechanics and Momentum Transfer

Using the result obtained from the graph, calculate the mass velocity of the gas as shown. 0:11gðρL - ρG ÞρG F P μ0:1 L ft lbm lbm lbm ð0:11Þð32:2 2 Þð62:4 3 - 0:0945 3 Þð0:0945 3 Þ sec m m m 110 0:1 ð Þð1:05cPÞ ft 2 = 0:4344 lbm=secft G=

=

Calculate the cross-section area and hence the diameter of the tower as shown. lbm 0:3155 G0 sec = 0:7263 ft2 = π D2 ) ACS = = lbm G 4 0:4344 2 sec‐ft D=

4 × 0:7263 ft2 = 0:9616 ft π

3.11

Flow Through Packed Beds

235

Generalized Pressure Drop Correlation (GPDC – S I Units)

Fig. 3.14 Generalized pressure drop correlation (SI units). (Generated by the author, N.S. Nandagopal, P E)

The nomenclature applicable to Fig. 3.14 is as follows: L′, V′: liquid and gas mass flow rates (kg/s) ρL, ρG: liquid and gas densities (kg/m3) μL: liquid viscosity (Pa  s) vG: superficial gas velocity (m/s) FP: packing factor (m-1)

3.11.2.2

Flooding in Packed Columns

Countercurrent flow packed columns, mainly used for gas absorption, can become flooded with liquid if either liquid or gas flow is too high. Flooding is accompanied by a dramatic increase in pressure drop, liquid hold-up, loss in separation efficiency, and potential damage to equipment. Extremely high gas velocities can result in pressure levels that could crush and damage the packings in the column. Higher gas velocities will increase the drag resistance encountered by the down-flowing liquid and hence the pressure drop across the packings. In determining the column diameter, we need to know the limiting (maximum) gas velocity that can be used. To understand the reasons for flooding, it is necessary to examine the relationship between the gas pressure drop and gas velocity in a packed column [2, 3]. With dry packing and no liquid flow (L = 0), pressure drop increases as gas velocity increases according to the linear relationship as shown by line 1-1 in Fig. 3.15. This is a straight line on a logarithmic plot. As liquid starts

236

3

Fluid Mechanics and Momentum Transfer

Fig. 3.15 Loading and flooding points in flows through packed columns

flowing in the column, part of the void volume in the packing is now filled with liquid (irrigated condition), thereby reducing the cross-sectional area available for gas flow. Hence, at the same gas velocity, the pressure drop is higher for wetted packings compared to dry packings. For any constant liquid flow (L = L1, L2, etc. shown in Fig. 3.15) at low to moderate gas velocity, the pressure drop characteristic is similar to that of dry packings, that is, section 2–3 of the plot which is still linear on the logarithmic plot of pressure drop vs. superficial mass velocity of the gas. Up to this point, there is an orderly trickling of the liquid down the packings. There is no observable liquid being trapped among the packings (no liquid hold-up). As the gas velocity is increased further, the pressure drop increases due to liquid hold up in the packing. At point 3, the liquid hold-up increases significantly resulting in an increase in the slope of the line at point 3 due to significantly higher pressure drop with increase in G. Point 3 is known as the loading point, since the liquid starts accumulating (“loading”) in the packing. Beyond point 3, at point 4 and further onto point 5, there is a greater amount of liquid hold-up, a gradual filling of the packing voids with liquid, beginning at the bottom of the column and the column is slowly “drowned” in the liquid, that is the column begins to flood at operating point 5, where there is another sharp increase in the slope (slope tends to infinity, that is, a vertical line). At this point the entire column is filled with liquid and the gas now must bubble through the liquid in the packing voids. The gas pressure drop is now very high. Point 5 is known as the flooding point. The gas velocity at this point is known as the flooding velocity, the upper limit of gas velocity. Based on the preceding discussion, the following conclusions can be drawn: • • • •

At a given liquid flow rate, gas pressure drop increases with gas velocity. At a given gas velocity, the gas pressure drop increases with liquid flow rate. Loading and flooding points are unique to each liquid flow rate. At higher liquid flow rates, the loading and flooding points occur at lower gas flow rates and hence at lower pressure drop.

Operation of a gas absorption column is not practical above the loading point. For optimum design, the recommended gas velocity is 1/2 of the flooding velocity. Alternatively, the design can be based on a specified pressure drop condition, usually well below the flooding point.

3.12

Fluidized Beds

237

3.12 Fluidized Beds Fluidization of a particulate bed occurs when there is sufficient force to lift the particles in the bed so that it behaves like a fluid, with the particles suspended in the flowing fluid. Fluidized beds have a wide range of applications in chemical processing. The most important are fluidized bed reactors, fluid catalytic cracking, fluidized bed combustion, and application of coatings onto solid particles [3, 6]. The condition where the particles just start lifting and moving from their fixed positions in the bed is called incipient fluidization and the velocity of the fluid in this condition is called minimum fluidization velocity, represented by vmf. Force balance under these conditions indicates that the buoyant force of the fluid is just equal to the weight of the bed. If ε is the porosity of the bed, then (1 - ε) is the fraction of the bed volume occupied by the solid particles. Therefore, the total weight of the particles in the bed is W P = ð1- εÞAHρP g

ð3:74Þ

In Eq. 3.74, A = cross-section area of the bed H = height of packing ρP = density of packing particle The upward thrust or the buoyant force experienced by the particles is the weight of the displaced fluid. F B = V fl:displ: ρf g = ð1- εÞAHρf g

ð3:75Þ

The net downward force experienced by the particles is balanced by the upward pressure force exerted by the fluid at the onset of fluidization. Therefore, this concept can be combined with the preceding equations to obtain the following result. W P - F B = ðΔPÞA ) ð1 - εÞAHρP g - ð1 - εÞAHρf g = ðΔPÞA ) ΔP = ð1- εÞHg ρP - ρf

ð3:76Þ

The pressure drop in the packed bed just prior to the incipient fluidization condition can be calculated using the Ergun equation (Eq. 3.70). Note: Appropriate use of the conversion constant, gc, is necessary in Eqns. 3.74–3.76 to ensure consistency of units. The incipient fluidization state is illustrated in Fig. 3.16. Example 3.22 The particles in a packed bed with a height of 3 ft have a density of 156 lbm/ft3. The cross-section area of the tower is 0.45 ft2. The total mass of particles in the bed is 120 lbm and the equivalent mean diameter of the particles is 0.04 in. The volume flow rate of the fluid (SG = 0.80, μ = 0.001 lbm/ft ‐ sec) across the bed is 0.0145 cfs (cubic feet per second). Determine

238

3

Fluid Mechanics and Momentum Transfer

Fig. 3.16 Incipient fluidization in a fluidized bed

A. the porosity of the bed. B. the pressure drop across the bed. C. the pressure drop at incipient fluidization. Solution A. Write Eq. 3.74 in terms of the mass of the particles and solve for the porosity of the bed by substituting the known values. mP = ð1 - εÞAHρP ) ð1 - εÞ =

mP = AHρP

120 lbm lbm 0:45 ft ð3 ftÞ 156 3 ft 2

) ε = 0:4302 ð ≃ 0:43Þ B. Convert the equivalent particle diameter to feet. DP =

0:04 in 12 in ft

= 0:0033 ft

Calculate the density of the liquid using its specific gravity. ρ = SG × ρw = ð0:80Þ 62:4

lbm = 49:92 lbm=ft3 ft3

= 0:5698

3.12

Fluidized Beds

239

Calculate the superficial velocity of liquid using Eq. 3.69. 3

vS =

ft 0:0145 sec Q = = 0:0322 ft= sec Acs 0:45 ft2

Substitute all the known values into Ergun Equation (Eq. 3.70) and solve for the pressure drop across the bed. Note that the Ergun equation in USCS units requires the use of the conversion constant gc to ensure consistency of units. ΔP 150vS μð1 - εÞ2 1:75ρv2S 1 - ε = þ DP gc ε3 H D2P ε3 gc ΔP = H

)

150vS μð1 - εÞ2 1:75ρv2S 1 - ε þ D P gc ε3 D2P ε3 gc ð150Þ 0:0322

ft sec

ð0:0033 ftÞ2 ð0:43Þ3 = ð3 ftÞ ð1:75Þ 49:92 þ

lbm ð1 - 0:43Þ2 ft‐ sec lbm‐ft= sec 2 32:2 lbf

0:001

lbm ft3

ð0:0033 ftÞ 32:2

2

ft ð1 - 0:43Þ 0:0322 sec

lbm‐ft= sec 2 ð0:43Þ3 lbf

= 187:2 lbf=ft2 Convert the pressure drop to psi. ΔP = 187

1 ft2 lbf × = 1:299 psi ft2 144 in2

C. Calculate the pressure drop at incipient fluidization using Eq. 3.76. (Note that the equation in USCS units requires the use of the conversion constant gc to ensure consistency of units.)

ΔP =

ð1 - εÞHg ρP - ρ f gc ft lbm lbm 156 3 - 49:92 3 2 sec ft ft lbm‐ft 32:2 lbf‐ sec 2

ð1 - 0:43Þð3 ftÞ 32:2 = = 181:4 lbf=ft2

240

3

Fluid Mechanics and Momentum Transfer

Convert the pressure drop at incipient fluidization to psi. ΔP = 181:4

3.13 3.13.1

lbf 1 ft2 × = 1:260 psi 2 144 in2 ft

Compressible Flow Introduction

Flow of gases through pipes and ducts [1, 5, 7] is also known as compressible flow since the density of the fluid is not constant during the flow. This is because the density of a gas is sensitive to pressure (unlike liquids) and hence keeps changing as pressure drop occurs during the flow. In contrast to incompressible flow, density and temperature are additional variables in compressible flow. Compressible flow is also known as variable density flow and the subject matter is sometimes referred to as gas dynamics [1, 5, 7]. At low velocities, gas flow can be considered incompressible. The criterion for gas flow to be incompressible is lower ranges of Mach number (defined as the ratio of the velocity of the fluid to the speed of sound). The density variations in gas flow are generally small (less than 5%), when the Mach number is less than 0.30 [7]. In chemical engineering applications, compressible flow is encountered in gas pipelines, and in flow of gases through nozzles and orifices, where the constricted flow area can result in high fluid velocities [3, 6]. The study of compressible fluid flow requires the knowledge and application of thermodynamic principles in addition to the knowledge base of fluid dynamics.

3.13.2

Continuity Equation for Compressible Flow

The law of conservation of mass must always be satisfied regardless of the type of flow or flow conditions. The continuity equation, applied to fluid flow, is based on the principle of conservation of mass, which is, the mass flow rate of the fluid remains constant throughout the flow field. For flow of liquids, it is reasonable to assume constant density and hence the volume flow rate is also conserved. However, in gas flow, since the density can vary in the flow field, the volume flow also varies (density is mass per unit volume and hence volume flow rate is not constant). Because of the variation of density with flow, the continuity equation for gas flow [1, 5, 7] must account for variation in density as shown in Eq. 3.77 (the same as Eq. 3.10). m_ = ρ1 A1 v1 = ρ2 A2 v2

ð3:77Þ

3.13

Compressible Flow

3.13.3

241

Mach Number and Its Significance in Compressible Flow

Consider the flow of gas from a source such as a compressor into a pipe. At steady state, the gas is already moving in the pipe at a constant velocity. As more gas gets into the pipe, it pushes the gas existing in the pipe and the pressure pulse results in a compression wave. The infinitesimally small pressure wave (disturbance) propagates through a fluid at the speed of sound [1, 5, 7]. If the fluid is incompressible, the pressure pulse will not have any effect on the fluid density since the density remains constant. In other words, the fluid adjacent to the pressure pulse will not get compressed and the compression wave is felt instantaneously throughout the pipe. If the fluid is compressible, then the pressure wave affects the fluid density by compressing (or piling up) the fluid layers adjacent to the wave. Thus, the speed of sound in any medium depends on the compressibility of the medium [1, 7]. The lower the compressibility of the medium, higher the speed of sound in the medium. In an ideal incompressible medium, the speed of sound approaches infinity. Solids have exceedingly small compressibility, whereas gases have relatively high compressibility. As a result, the speed of sound in solids will be typically 10–15 times higher than in a gas at ambient temperature. The local speed of sound in a gas depends only on the absolute temperature of the gas, and it is calculated by using Eqs. 3.78a and 3.78b. p SI units c = kRT USCS units

c=

kRTgc

ð3:78aÞ ð3:78bÞ

In Eqs. 3.78a and 3.78b, k is the ratio of specific heats of the gas (k = cp/cv), R is the individual gas constant, and T is the absolute temperature of the gas. The Mach number, Ma, is the ratio of the average fluid velocity to the local speed of sound in the gas. Ma =

v c

ð3:79Þ

Since the pressure disturbance in a gas propagates at the speed of sound, it follows that the Mach number has a significant impact on the nature of flow. The physical significance of the Mach number can be summarized as follows [1, 5, 7]: 1. If Ma < 0.3, then the flow can be considered incompressible. 2. The Mach number is proportional to the ratio of kinetic energy of the fluid to the internal energy of the fluid as shown here (from thermodynamics, the individual gas constant, R = cp–cv). Ma 

kinetic energy v2 v2 v v v2 α α α p α c k ðk - 1Þu kRT kRT k cp - cv T k ðk - 1Þcv T

242

3

Fluid Mechanics and Momentum Transfer

In the preceding equation u is the specific internal energy of the fluid. The Mach number is the single most important parameter in understanding and analyzing compressible flows [1, 5, 7]. If the velocity of the fluid is greater than the velocity of sound (Ma > 1.0) the flow is characterized as supersonic flow. If the velocity of the fluid is less than the velocity of sound (Ma < 1.0), the flow is characterized as subsonic flow. Flows with Ma = 1.0 are characterized as sonic flows.

3.13.4

Isentropic Gas Flow

During isentropic flow of an ideal gas in a duct, pipe, or nozzle, the entropy remains constant [1, 5, 7]. Since an isentropic process is a reversible adiabatic process, there is no heat transfer between the gas and the surroundings. The following P-V-T relationships are applicable for isentropic processes. v P2 = 1 P1 v2

k

=

ρ2 ρ1

P2 T2 = P1 T1 ρ2 P2 = ρ1 P1

1 k

=

T2 T1

k

since ρ =

1 v

ð3:80Þ

k k-1

k k-1

ð3:81Þ 1 k

=

T2 T1

1 k-1

ð3:82Þ

Equation 3.80 is particularly useful for compressible fluid flow where the density varies in the flow field.

3.13.5

Application of the Steady Flow Energy Equation for Isentropic Flows

The energy relationship between two points in the isentropic flow field (no heat transfer, no work, no elevation change, only enthalpy and kinetic energy terms need to be considered in the steady flow energy equation) can be obtained by energy balance between the two points. Energy in at reference point 1 = Energy out at reference point 2 h1 þ

v1 2 v2 = h2 þ 2 2 2

ð3:83Þ

3.13

Compressible Flow

243

Example 3.23 Air flows in an adiabatic divergent nozzle at a velocity of 560 ft/sec. The temperature of air at this location is 80 °F. At the divergent section, the air speed reduces to 210 ft/s. The specific heat of air at constant pressure is cp = 0.24 Btu/lbm ‐ ° R. Calculate the temperature of air at the divergent section. Solution The kinetic energy will decrease when the air speed is reduced. However, since the total energy is conserved, the enthalpy, and hence the temperature must increase to compensate for the loss in kinetic energy. Let reference point 1 represent the normal section of the nozzle and reference point 2 represent the divergent section of the nozzle. The absolute temperature at reference point 1 is T1 = 460 ° + 80 ° F = 540 ° R Apply Eq. 3.83 (include the conversion constants, gc and j, to reconcile the units) between reference points 1 and 2, simplify, and substitute the known values to obtain the temperature of air at the divergent section. v21 v2 = h2 þ 2 ) 2gc j 2gc j 2 2 v - v2 h 2 - h1 = 1 ) 2gc j v2 - v22 cp ð T 2 - T 1 Þ = 1 ) 2gc j h1 þ

T2 = T1 þ

v21 - v22 2cp gc j 2

= 540 ° R þ

ft ft - 210 sec 560 sec Btu lbm‐ft ð2Þ 0:24 32:2 lbm‐ ° R lbf‐ sec 2

2

778 ft‐lbf Btu

= 562:4 ° R ð102:4 ° FÞ

3.13.6

Stagnation – Static Relationships

Consider a compressed gas in a reservoir. Since the velocity of the gas is zero in the reservoir, the conditions in the reservoir are referred to as stagnation (zero velocity) conditions. Stagnation conditions (P0, T0, ρ0, s0, h0) are identified with subscript “0” and they can occur at any point in the flow field. If the reservoir is connected to a pipeline, the gas starts flowing in the pipeline and the conditions within the pipeline flow field are static conditions (P, T, ρ, s, h) and they are represented as variables

244

3

Fluid Mechanics and Momentum Transfer

without subscripts. Apply Eq. 3.83 between the stagnation point (v0 = 0) and another point in the isentropic flow field and use the relationship h = cpT for an ideal gas to obtain the relationship in Eq. 3.84 [1, 5, 7].

v2 T0 =1 þ T 2cp T

ð3:84Þ

The preceding equation can be written in terms of Mach number and simplified by using the following relationships to obtain Eq. 3.85. p v2 kR = Ma2 , c = kRT , cp = k-1 c2 k-1 T0 Ma2 =1 þ T 2

ð3:85Þ

Equation 3.85 can be combined with the P-V-T relationships for isentropic processes to obtain additional stagnation – static relationships for isentropic flow.

3.13.7

k-1 P0 Ma2 = 1þ P 2

k k-1

ρ0 k-1 Ma2 = 1þ ρ 2

1 k-1

ð3:86Þ ð3:87Þ

Isentropic Flow with Area Changes

Any type of fluid flow must satisfy both the continuity and momentum equations. In addition, compressible flows must satisfy the energy equation and the relevant P-V-T relationships. Applying the preceding principles to isentropic flows with area changes, it can be shown that for subsonic flows (Ma < 1.0), decrease in flow area in the flow direction results in increase in velocity whereas for supersonic flows (Ma > 1.0), increase in flow area in the flow direction is required for the velocity to increase [1, 7]. Compressible flows are often accelerated or decelerated through a nozzle or diffuser. The flow is sonic when Mach number is one and the point at which sonic flow occurs is known as the critical point or throat [1, 5, 7]. The throat area is

3.13

Compressible Flow

245

represented by the symbol, A*. The ratio between the critical temperature and the stagnation temperature can be obtained by substituting Ma = 1 in Eq. 3.85 as shown here. T0 k-1 =1 þ Ma2 T 2

Ma = 1, T = T

→

T 2 = T0 kþ1

ð3:88Þ

Substituting k = 1.4 for air into the preceding equation, the critical temperature ratio for air is 2 T = 0:8333 = 1:4 þ 1 T0

ð3:89Þ

Equation 3.89 implies that for air, sonic flow occurs when the temperature in the duct reaches 83% of the stagnation temperature. Similar critical ratios can be obtained by using isentropic P-V-T relationships as shown. P T = P0 T0

k k-1

ρ T = ρ0 T0

1 k-1

=

2 kþ1

=

2 kþ1

k k-1

1 k-1

=

2 1:4 þ 1

=

2 1:4 þ 1

1:4 1:4 - 1

1 1:4 - 1

= 0:5283

ð3:90Þ

= 0:6339

ð3:91Þ

Once sonic velocity is achieved in a duct or a nozzle throat, the mass flow rate remains constant, and it is at its maximum value and the flow condition is referred to as choked flow. The mass flow rate for compressible flow can be written in terms of Mach number as shown here. m_ = ρAv =

P AðMaÞðcÞ = RT

p P AðMaÞ kRT RT

= PAðMaÞ

ð3:92Þ

k RT

In Eq. 3.92, substitute for T from Eq. 3.85 and for P from Eq. 3.86 to obtain the following equation for compressible flow mass flow rate. m_ = ðP0 AÞðMaÞ

k RT 0

k-1 1þ Ma2 2

kþ1 2ð1 - kÞ

ð3:93Þ

246

3

Fluid Mechanics and Momentum Transfer

For sonic flow, Ma = 1.0 and the area at the sonic point is A*. Substitute Ma = 1.0 into the preceding equation. k RT 0

m_ = ðP0 A Þ

1þ

kþ1 2ð 1 - k Þ

k-1 2

ð3:94Þ

Equate the right-hand sides of Eqs. 3.93 and 3.94 and simplify to obtain the following equation for the area ratio between the throat area and the area at any location in the flow field [1, 5, 7]. A 1 2 þ ðk - 1ÞMa2  = kþ1 Ma A

kþ1 2ðk - 1Þ

ð3:95Þ

Example 3.24 Air flows from a process tank maintained at 275 °C and 20 bar into a convergent– divergent nozzle. At a section where the flow area is 50% of the flow area at the discharge of the divergent section, the Mach number is 0.75. Determine: A. the pressure, temperature, linear velocity, density, and mass velocity at the narrow section, where the area is 50% of the exit area. B. the values of all the variables under sonic (Ma = 1.0) conditions. Solution Designate the narrow section as “Section 2.” Convert the static temperature to its absolute value, T 0 = 273 ° þ 275 ° C = 548 K A. Calculate the temperature at Section 2 using Eq. 3.85. k-1 T0 = 1þ Ma2 ) T 2 548 K T0 = = 493 K ð220 ° CÞ T= k-1 1:4 -1 2 2 1þ Ma 1þ 0:75 2 2 Calculate the pressure at Section 2 using Eq. 3.86. P0 = 1þ P P= 1þ

k-1 2

Ma2

k k-1

P0 k-1 2

Ma2

k k-1

) =

20 bar 1þ

1:4 - 1 2

0:752

1:4 1:4 - 1

= 13:77 bar

3.13

Compressible Flow

247

Calculate the gas constant for air. 3

R=

barm 0:08314 kmolK R = 0:0029 bar  m3 =kg  K = kg M 29 kmol

Calculate the static density of air at the tank pressure and temperature using the ideal gas law. ρ0 =

P0 20 bar = = 12:58 kg=m3 3 RT 0 0:0029 barm ð 548 K Þ kgK

Calculate the density at Section 2 using Eq. 3.87. ρ0 = 1þ ρ ρ=

k-1 2

Ma2

1 k-1

ρ0 1þ

k-1 2

Ma2

1 k-1

) =

kg m3 0:752

12:58 1þ

1:4 - 1 2

1 1:4 - 1

= 9:64 kg=m3

Calculate the speed of sound at Section 2 using Eq. 3.78a. p c = kRT =

Note on units:

J kmolK × K kg kmol



J 8314 kmolK

ð1:4Þ

Nm kg

kg 29 kmol



kgm m s2

kg

ð493 KÞ = 444:83 m=s

= m=s

Calculate the linear velocity at Section 2 using the Mach number and the speed of sound at the throat. v ) c m v = ðMaÞðcÞ = ð0:75Þ 444:83 = 333:62 m=s s

Ma =

Calculate the mass velocity at Section 2 using the linear velocity and the density at the throat. G = vρ = 333:62

m s

9:64

kg = 3216 kg=m2  s m3

248

3

Fluid Mechanics and Momentum Transfer

B. Calculate the values of the temperature, pressure, and density under sonic (Ma = 1.0) conditions using Eqs. 3.89, 3.90, and 3.91 respectively (applicable for air) as shown. T = 0:8333 ) T0  T = 0:8333T 0 = ð0:8333Þð548 KÞ = 457 K P = 0:5283 ) P0  P = 0:5283P0 = ð0:5283Þð20 barÞ = 10:57 bar ρ = 0:6339 ) ρ0 ρ = 0:6339ρ0 = ð0:6339Þ 12:58

kg = 7:97 kg=m3 m3

Calculate the speed of sound at the sonic condition Eq. 3.78a. p c = kRT  =

ð1:4Þ

J 8314 kmolK kg 29 kmol

ð457 KÞ = 428:28 m=s

Since Ma* = 1 at sonic condition, the linear velocity at sonic condition is v = c = 428:28 m=s Calculate the mass velocity at sonic condition using the linear velocity and the density at sonic condition. G = v ρ = 428:28

3.13.8

m s

7:97

kg = 3413 kg=m2  s m3

Adiabatic Compressible Flow with Friction Loss

The Fanning friction factor can also be used for compressible flow. Let L* be the length of the duct required for the flow to develop from given Mach number to sonic condition (Ma = 1) and if f is the average Fanning friction factor between L = 0 and L = L*, then the following equation can obtained by applying the principles of conservation of mass, momentum, and energy [1, 5, 7].

3.13

Compressible Flow

249

ðk þ 1ÞMa2 kþ1 4f L 1 - Ma2 ln þ = 2k D kMa2 2 þ ðk - 1ÞMa2

ð3:96Þ

If ΔL is the length between the locations of Ma1 and Ma2, then the following relationship can be written. 4f

ΔL 4f L = D D

1

4f L D

ð3:97Þ 2

The following equations can be written for the ratios between flow variables along the duct [1, 7]. 1 kþ1 P = P Ma 2 þ ðk - 1ÞMa2 T c =  T c

2

=

1 2

kþ1 2 þ ðk - 1ÞMa2

ð3:98Þ ð3:99Þ

Example 3.25 illustrates the use of the preceding equations in adiabatic compressible flow with friction loss. Example 3.25 Air flows through a circular duct (ID = 102 mm). The pressure, temperature, and Mach number at Section 1 of the duct are: P1 = 800 kPa, T 1 = 147 ° C, Ma1 = 0:17: The average Fanning friction factor for flow in this duct can be taken as 0.006. Determine the Mach number, pressure, and temperature 35 m downstream from Section 1. Solution Substitute the known values into Eq. 3.96 and calculate the parameter 4f L =D at section 1 as shown. 4fL D

1 - Ma1 2 kþ1 ðk þ 1ÞMa1 2 þ ln 2 2k kMa1 2 þ ðk - 1ÞMa1 2 1 2 1:4 þ 1 ð1:4 þ 1Þ0:172 1 - 0:17 þ ln = 2 × 1:4 1:4 × 0:172 2 þ ð1:4 - 1Þ0:172 = 21:11 =

Substitute the known values into Eq. 3.97 and obtain the value for the parameter 4f L =D at Section 2 as shown.

250

3

4fL D

= 2

4fL D

Fluid Mechanics and Momentum Transfer

ΔL - 4f D 1

= 21:11 - 4ð0:006Þ

35 m 102 mm 1000 mm 1m

= 12:87

Substitute Ma2 for the Mach number into Eqn. 3.96 and solve for Ma2 using the preceding value obtained for 4f L =D 2 . Note: For subsonic flow, as is the case here, Mach number increases downstream of the initial section. 4f L D

= 2

ðk þ 1ÞMa2 2 k = 1:4 1 - Ma2 2 kþ1 þ ln = 12:87 → 2 2 2k kMa2 2 þ ðk - 1ÞMa2

0:7143

1 2:4Ma2 2 - 1 þ ð0:8571Þ ln = 12:87 2 Ma2 2 þ 0:4Ma2 2

After solving the preceding non-linear equation for Ma2 by trial and error, Ma2 = 0.21. Derive an expression for the pressure at Section 2 by using pressure ratios involving sonic pressures at sections 1 and 2, which are equal. P2 = P1

P1  P1

P2  P1 

P2 P1  P2 ð1Þ  = P1 P2 P1 P2 

Use Eq. 3.98 to obtain the values for the ratios between sonic pressure and static pressure at sections 1 and 2 as shown here. 1

2 1 1 1:4 þ 1 kþ1 P1 =  = 2 Ma1 2 þ ðk - 1ÞMa1 0:17 2 þ ð1:4 - 1Þ0:172 P1 P  1 = 6:425 ) 1 = = 0:1556 P1 6:425

kþ1 P2 1 = P2  Ma2 2 þ ðk - 1ÞMa2 2

1 2

1 1:4 þ 1 = 0:21 2 þ ð1:4 - 1Þ0:212

Substitute the preceding results into the equation derived for P2.

1 2

1 2

= 5:19

Practice Problems

P2 = P1

251

P2 P1  ð 1Þ = ð800 kPaÞð0:1556Þð1Þð5:19Þ = 646 kPa P1 P2 

In a similar manner, obtain the temperature at Section 2 by using temperature ratios. The absolute temperature at Section 1 is T1 = 273 ° + 147 ° C = 420 K T2 = T1

T 1 T1

T 2 T 1

T 1 T2 T2 ð 1Þ  = ð420 KÞ T2 T1 T 2

Use Eq. 3.99 to obtain the values for the ratios between sonic temperature and static temperature at sections 1 and 2 as shown here. T1 kþ1 1:4 þ 1 T  = = 1:193 ) 1 = 0:8382  = 2 2 T1 T1 2 þ ðk - 1ÞMa1 2 þ ð1:4 - 1Þ0:17 kþ1 1:4 þ 1 T2 = = = 1:189 T 2  2 þ ðk - 1ÞMa2 2 2 þ ð1:4 - 1Þ0:212 Substitute the preceding results into the equation for T2. T 2 = ð420 KÞð0:8382Þð1Þð1:189Þ = 418:6 K

Practice Problems Practice Problem 3.1 An oil has specific gravity (SG) of 0.90 and absolute viscosity of 15 cP. Calculate: A. B. C. D.

the density of the oil in lbm/ft3 and kg/m3. the specific weight of the oil in lbf/ft3 and N/m3. the dynamic viscosity in lbf-sec/ft2, lbm/ft-sec, N.s/m2, and kg/m.s. the kinematic viscosity in m2/s, ft2/sec, and centistokes.

Practice Problem 3.2 An inclined manometer is used in measuring the pressure difference between the two water tanks, A and B, as shown. The specific gravity of mercury is 13.6. Calculate the pressure difference in psi.

252

3

Fluid Mechanics and Momentum Transfer

Practice Problem 3.3 200 gpm of water at 70 °F flows in a steel pipe with nominal diameter of 2 in (ID = 2.07 in). It is proposed to increase the velocity of water by at least 20% by using a smaller pipe. Determine the nearest standard pipe size required to achieve the specified increase in velocity.

Practice Problem 3.4 100 gpm of water at 70 °F flows in a steel pipe with nominal diameter of 2 in (ID = 2.07 in). At 70 °F, the properties of water are ρ = 62.3 lbm/ft3 and μ = 2.05 × 10-5 lbf-sec/ft2. Calculate the Reynolds number.

Practice Problem 3.5 Using the same data as in Practice Problem 3.4, determine A. the Fanning friction factor. B. the pressure drop (psi) across 100 ft of this pipe.

Practice Problems

253

Practice Problem 3.6 A bypass piping network is shown in the figure below. The velocities and the Fanning friction factors will be approximately equal in the straight pipe as well as in the bypass loop, while either one of them is in operation. Determine the required ratio of the diameter of the straight pipe to the diameter of the bypass loop to maintain identical pressure loss in the two systems, while each is in operation.

Practice Problem 3.7 A process liquid with specific gravity 1.13 is flowing in a 6 in pipe (ID = 6.065 in). The liquid flow is monitored using an orifice meter with a diameter of 2.4 in. A differential pressure gage attached across the meter shows a reading of 3 psig. Determine: A. the reading of a differential mercury manometer used in place of a differential pressure gage. B. the flow rate of the liquid in gallons per minute (gpm).

Practice Problem 3.8 A venturi meter is used in measuring the flow rate of oil (SG = 0.88) in a pipe with ID = 100 mm. The throat diameter of the Venturi meter is 50 mm. If the flow rate of oil is 25.24 L/s, determine the reading of a differential pressure gage attached across the venturi meter in bar(g).

Practice Problem 3.9 Determine the Reynolds number for the flow of 15 L/min of water (ρ = 1000 kg/m3, μ = 1 cP) through a packed bed (porosity 35%) in a column with diameter 65 cm and consisting of 5 mm diameter spherical particles.

254

3

Fluid Mechanics and Momentum Transfer

Practice Problem 3.10 A pollutant is to be removed from air using water as the solvent in a 40 cm diameter packed tower. The gas flow rate is 0.35 kg/s. The packing used is Super Intalox Plastic Saddle #1. The design pressure drop is such that flooding can be avoided. The relevant fluid properties and packing characteristics are: ρL = 1000 kg=m3 , μL = 1:08 cP, ρG = 1:53 kg=m3 Packing factor, FP = 100 m-1. Determine the liquid to gas mass flow ratio.

Practice Problem 3.11 A packed bed consists of 195 kg of spherical particles with diameter 0.055 m and density 1800 kg/m3. The bed is fluidized by air at a volume flow rate of 2.67 m3/min. The properties of air are ρf = 1.18 kg/m3, μ = 1.89 × 10-5Pa  s. The vessel diameter is 0.45 m and the bed porosity at incipient fluidization is 0.48. Determine A. the bed height at incipient fluidization. B. the minimum fluidization velocity.

Practice Problem 3.12 Air flows through a convergent–divergent nozzle. When the Mach number is 0.88, the nozzle area is 225 in2. The initial pressure and temperature of air are 75 psia and 250 °F, respectively. Determine: A. the throat area required to achieve sonic velocity. B. the maximum mass flow rate of air when the flow is choked.

Solutions to Practice Problems Practice Problem 3.1 Solution Use the definition of specific gravity to obtain the density and specific weight of the oil.

Solutions to Practice Problems

255

A. From Eq. 3.1,

ρoil = ðSGoil Þðρwater Þ = ð0:90Þ 62:4

lbm ft3

= 56:16 lbm=ft3 ρoil = ðSGÞðρwater Þ = ð0:90Þ 1000

kg m3

= 900 kg=m3 B. From Eq. 3.1,

γ oil = ðSGÞðγ water Þ = ð0:90Þ 62:4 = 56:16 lbf=ft3 γ oil = ðSGÞðγ water Þ = ð0:90Þ 9:81

lbf ft3 kN m3

= 8:83 kN=m3 C. Given that absolute or dynamic viscosity, μ = 15 cP. Use the conversion factors to obtain the dynamic viscosity in the desired units.

μ = ð15 cPÞ μ = ð15 cPÞ

μ = ð15 cPÞ

0:000021 lbf‐ft2sec 1 cP 0:000672 ft‐lbm sec 1 cP Ns m2 1 cP

0:001

= 0:00032 lbf‐ sec=ft2 = 0:0101 lbm=ft‐ sec

= 0:015 N  s=m2 ðPa:sÞ = 0:015 kg=m  s

D. Calculate the kinematic viscosity by using Eq. 3.3.

ν=

μ 0:0101ft‐lbm sec = 1:798 × 10 - 4 ft2 = sec = lbm ρ 56:16 3 ft

256

3

ν=

Fluid Mechanics and Momentum Transfer

kg μ 0:015 m:s = 1:667 × 10 - 5 m2 =s = ρ 900 mkg3

Calculate the kinematic viscosity in centistokes using the relevant conversion factor. ν = 1:667 × 10 - 5

m2 s

1 cSt 2 1:0 × 10 - 6 ms

= 16:67 cSt

Practice Problem 3.2 Solution Designate the vertical distances as shown in the figure and convert them to feet.

hw1 =

0:80 in

hw2 =

0:90 in

hHg =

12 in 1 ft

12 in 1 ft

= 0:067 ft = 0:075 ft

1:80 in × sin 30 ° 12 in 1 ft

= 0:075 ft

Solutions to Practice Problems

257

Use the principles of static pressure to obtain the pressures at points 1 and 3. P1 = PA þ γ w hw1 P3 = PB þ γ w hw2 þ γ Hg hHg Since pressures are equal at the same level in the same fluid, P1 = P3 PA þ γ w hw1 = PB þ γ w hw2 þ γ Hg hHg Calculate the required pressure difference by using the preceding equation. PA - PB = γ w ðhw2 - hw1 Þ þ γ Hg hHg lbf ð0:075 ft - 0:067 ftÞ ft3 lbf þ 13:6 × 62:4 3 ð0:075 ftÞ ft = 64:15 lbf=ft2 ð0:4455 psiaÞ = 62:4

Practice Problem 3.3 Solution Subscript “1” refers to the original situation and subscript “2” refers to the new situation with 20% increase in velocity. Use the continuity equation (Eq. 3.9), with the volume flow remaining constant. Q = v1 A1 = v2 A2 Therefore, v2 A1 = = v1 A2

π 4 π 4

D1 2 D1 2 = D2 2 D2 2

Solve the preceding equation for D2 and substitute the known values of the ratio v2/v1 and D1. D2 =

D1 v2 v1

2:07 in = 1:89 in = p 1:20

258

3

Fluid Mechanics and Momentum Transfer

From the pipe dimension tables, the nearest standard size is 1.5 in nominal diameter with ID = 1.61 in, as shown in excerpt from the pipe data table. With this size, the velocity will increase by more than 20%, which is acceptable.

Practice Problem 3.4 Solution Convert the pipe ID to feet. D=

2:07 in = 0:1725 ft 12 in ft

Calculate the velocity of water by using the continuity equation (Eq. 3.9). ft3 sec 100 gpm × 449 gpm 1

v=

Q = A

π 4 ð0:1725

ftÞ2

= 9:53 ft= sec

From Eq. 3.6, ft lbm 62:3 3 sec Dvρ ft Re = = μ lbm‐ft lbf‐ sec 2:05 × 10 - 5 32:2 lbf‐ sec 2 ft2 = 1:55 × 105 ð0:1725 ftÞ 9:53

Solutions to Practice Problems

259

Practice Problem 3.5 Solution The following data and results from Practice Problem 3.4 are reproduced here for reference. D = 0:1725 ft ð2:07 inÞ, ρ = 62:3 lbm=ft3 , μ = 2:05 × 10

-5

2

lbf‐ sec=ft ,

v = 9:53 ft= sec

Re = 1:55 × 105

From Fig. 3.6, the roughness of a steel pipe is ε = 0.0018 in. Calculate the relative roughness using Eq. 3.16. r=

ε 0:0018 in = = 0:0009 D 2:07 in

With Re and r as parameters, determine the Fanning friction factor from Fig. 3.6 as shown.

From the figure shown, the Fanning friction factor is, f = 0.0058. B. Calculate the friction head loss using Eq. 3.14.

hf =

2fLv2 ð2Þð0:0058Þð100 ftÞ 9:53 fts = gD 32:2 sft2 ð0:1725 ftÞ

2

= 18:97 ft

Calculate the pressure drop in psi by using the pressure equivalent, 1 psi = 2.31 ft of water. ΔP =

18:97 ft water = 8:21 psi 2:31 ft water 1 psi

260

3

Fluid Mechanics and Momentum Transfer

Practice Problem 3.6 Solution Subscript “s” represents the straight pipe and subscript “b” represents the bypass loop. Since the pressure drops must be identical, the head loss across the straight pipe must be equal to the head loss across the bypass loop. Simplify Eq. 3.32 to get the required ratio (Ds/Db), and substitute the known values.

Practice Problem 3.7 Solution A. Calculate the reading of a differential mercury manometer using Eq. 3.6.

hm =

ΔP = γm - γf

lbf 3in 2 ×

144 in2 ft2

lbf lbf 13:6 × 62:4 3 - 1:13 × 62:4 3 ft ft

= 0:5552 ft ð6:66 inÞ

B. Calculate the cross section area of the orifice. 2

Ao =

π 2 π D = 4 o 4

2:4 in 12 in ft

= 0:0314 ft2

Calculate the volume flow rate by using Eq. 3.58.

Solutions to Practice Problems

Q = C o Ao

261

ΔP γw

2g

= ð0:61Þ 0:0314 ft2

ft s2

ð2Þ 32:2

3

lbf 144 in2 × in2 ft2 lbf 1:13 × 62:4 ft3

= 0:3805 ft3 =s Convert the volume flow rate to gallons per minute (gpm). Q = 0:3805

ft3 449 gpm = 170:84 gpm × ft3 sec sec

Practice Problem 3.8 Solution Calculate the ratio of the throat diameter to the pipe diameter. β=

D2 50 mm = = 0:50 D1 100 mm

Calculate the cross section area of the throat. 2

A2 =

πD2 2 4

50 mm π 1000 mm 1m = 4

= 0:001963 m2

Using the given specific gravity of the oil, calculate its specific weight. γ = ðSGÞγ w,std = 0:88 × 9:81

kN = 8:633 kN=m3 m3

262

3

Fluid Mechanics and Momentum Transfer

Convert the volume flow rate into m3/ s. Q = 25:24

L 1 m3 = 0:02524 m3 =s × s 1000 L

Solve Eq. 3.64 for ΔP and calculate the pressure drop. Q=

C v A2

ΔP γ

2g

1 - β4

) 2g 3

Q 1-β γ 2

ΔP =

4

2gðC v A2 Þ2

=

0:02524 ms

Q2 1 - β 4 ΔP = ) γ ðCv A2 Þ2 2

1 - 0:54

8:633

kN m3

m ð0:98 × 0:001963 m2 Þ2 s2 = 71:01 kN=m2 ðkPaÞ 2 × 9:81

Convert the pressure drop to bar(g). ΔP = 71:01 kPa ×

1 bar = 0:7101 barðgÞ 100 kPa

Practice Problem 3.9 Solution Calculate the superficial velocity of water using Eq. 3.69. L × 15 min Q vS = = Acs π 4

1 min 60 s 65 cm

×

1 m3 1000 L

2

= 0:00075 m=s

100 cm m

Calculate the Reynolds number using Eq. 3.72. D vρ Re P = P S = ð1 - εÞμ

5 mm 1000 mm m

0:00075 ms 1000 mkg3

ð1 - 0:35Þ 1 cp ×

10 - 3 N:s2 m cp

= 5:77

Solutions to Practice Problems

263

Practice Problem 3.10 Solution Calculate the superficial velocity of air using the continuity equation (Eq. 3.10). m_ G m_ G = ρG AvG ) vG = = ρG A

0:35 kg 1:53 3 m

π 4

kg s 40 cm

2

= 1:820 m=s

100 cm 1m

Calculate the parameter on the y-axis as shown.

v2G ρG μ0:1 L FP ρL - ρG

=

ð1:820 ms Þ2 1:53

kg 1 Pa:s 0:1 Þ 100 ð1:08 cP × 0:001 1cP 3 m m kg kg 1000 3 - 1:53 3 m m

= 0:2563 ð0:26 × 10 - 1 Þ Locate 0.26 (2.6 × 10-1) on the y-axis and draw a horizontal line from there to intersect the maximum pressure drop curve below the flooding curve as shown in the figure. From the intersection point, draw a vertical line to the x-axis to get the value 0.02 for the x-axis parameter.

264

3

Fluid Mechanics and Momentum Transfer

Therefore, L0 G0

ρG L0 = 0:02 ) 0 = ð0:02Þ ρL G

ρL = ð0:02Þ ρG

1000 mkg3 1:53 mkg3

= 0:51

The liquid to gas mass flow ratio is .0.51. The liquid to gas mass flow ratio from the GPDC graph is 0.51. Since the pressure drop curve that was used to obtain this result is close to the flooding curve, even lower liquid to gas mass flow ratio, such as 0.45, can be used for conservative purposes.

Practice Problem 3.11 Solution A. At incipient fluidization, the weight of the particles is balanced by the buoyant force of the fluid. Write Eq. 3.74 in terms of the mass of the particles and solve for the height of the bed by substituting the known values. mP = ð1 - εÞAHρP ) mP H= = ð1 - εÞAρP ð1 - 0:48Þ

195 kg π ð0:45 mÞ2 4

1800

kg m3

= 1:31 m

B. Calculate the pressure drop at incipient fluidization using Eq. 3.76. ΔP = ð1 - εÞHg ρP - ρ f = ð1 - 0:48Þð1:31 mÞ 9:81 = 12021 Pa ð ≃ 12:02 kPaÞ

m s2

1800

kg kg - 1:18 3 m m3

N Note on units: skgm 2 m2  m2  Pa

Use the preceding pressure drop in the Ergun equation and solve the resulting quadratic equation for the minimum fluidization velocity.

Solutions to Practice Problems

ΔP = H

265

150vmf μð1 - εÞ2 1:75ρv2mf 1 - ε þ DP ε3 D2P ε3

)

ð150Þðvmf Þ 1:89 × 10 - 5 Pa:s ð1 - 0:48Þ2 ð0:055 mÞ2 ð0:48Þ3 12021 Pa = ð1:31 mÞ

ð1:75Þ 1:18 þ

kg ðvmf Þ2 ð1 - 0:48Þ m3

)

ð0:055 mÞð0:48Þ3

0:01923v2mf þ 0:00025vmf - 1 = 0 ) vmf = 7:20 m=s

Practice Problem 3.12 Solution A. Solve Eq. 3.95 for the throat area, A*, and substitute the known values to obtain the throat area. A 1 2 þ ðk - 1ÞMa2 = kþ1 A Ma A = ðA × MaÞ

2þðk - 1ÞMa2 kþ1

= 225 in2 × 0:88

kþ1 2ðk - 1Þ

)

kþ1 2ð 1 - k Þ

2þð1:4 - 1Þ0:882 1:4þ1

1:4þ1 2ð1 - 1:4Þ

= 222 in2 Alternative Solution Using Isentropic Compressible Flow Functions for Air Determine the ratio A/A* from the table as shown here. Isentropic compressible – Flow functions M 0.86 0.87 0.88 0.89 0.90

A/A* 1.0179 1.0153 1.0129 1.0108 1.0089

266

3

Fluid Mechanics and Momentum Transfer

A A 225 in2  = 222 in2 =  = 1:0129 ) A = 1:0129 1:0129 A B. Calculate the absolute value of the initial temperature. T 0 = 460 ° þ 250 ° F = 710 ° R Calculate the individual gas constant of air. R=

ft‐lbf 1545 lbmol R - °R = 53:28 ft‐lbf=lbm - ° R = lbm M 29 lbmol

Calculate the maximum mass flow rate using Eq. 3.94 for choked flow. Note that for reconciling the units in USCS the conversion constant gc needs to be used in the formula as shown. kgc RT 0

m_ = ðP0 A Þ

lbf = 75 2 in

222 in

1þ

2

k-1 2

kþ1 2ð 1 - k Þ

1:4 × 32:2 53:28

lbm‐ft lbf‐ sec 2

ft‐lbf × 710 ° R lbm‐ ° R 1þ

1:4 - 1 2

×

1:4þ1 2ð1 - 1:4Þ

= 332:62 lbm= sec Alternate Solution Calculate the sonic temperature using Eq. 3.89. T  = 0:8333T 0 = 0:8333 × 710 ° R = 591:6 ° R Calculate the sonic pressure using Eq. 3.90. P = 0:5283T 0 = 0:5283 × 75 psia = 39:62 psia Since the flow is choked, Ma = 1.0. Calculate the sonic velocity using Eq. 3.78b. v = c = =

kRT  gc ð1:4Þ 53:28

= 1192 ft= sec

ft‐lbf lbm‐ft ð591:6 ° RÞ 32:2 lbm‐ ° R lbf‐ sec 2

References

267

Determine the individual gas constant for air. R=

10:73 R = M 29

psia‐ft3 lbmol‐ ° R lb lbmol

= 0:3700 psia‐ft3 =lbm ° - R

Calculate the mass flow rate at sonic conditions using the continuity equation. ft 1 ft2 222 in2 × Pv A sec 144 in2 m  = ρ v A  =  = RT psia‐ft3 ð591:6 ° RÞ 0:37 lbm‐ ° R = 332:62 lbm= sec   

ð39:62 psiaÞ 1192

References 1. G. Bar-Meir, Fundamentals of Compressible Flow Mechanics. GNU Free Documentation License (2013). Download from https://mountainscholar.org/bitstream/handle/20.500.11785/2 61/OTL_BookId 86_gasDynamics.pdf?sequence=1 2. J. Mackowiak, Fluid Dynamics of Packed Columns: Principles of the Fluid Dynamic Design of Columns for Gas/Liquid and Liquid/Liquid Systems (Springer, Heidelberg/Berlin, 2010) 3. W.L. McCabe, J.C. Smith, P. Harriott, Unit Operations of Chemical Engineering, 7th edn. (McGraw Hill, New York, 2021) 4. J.W. Mitchell, Fox and McDonald’s Introduction to Fluid Mechanics, 10th edn. (Wiley, Hoboken, 2020) 5. N.S. Nandagopal, Fluid and Thermal Sciences – A Practical Approach for Students and Professionals (Springer Nature, Cham, 2022) 6. N.D. Nevers, G.D. Silcox, Fluid Mechanics for Chemical Engineers, 4th edn. (McGraw Hill, New York, 2021) 7. E. Pardyjak, Introduction to Compressible Flow. University of Utah, Mechanical Engineering Department (2018). Download from https://my.mech.utah.edu/~pardyjak/me3700/ IntroCompFlow.pdf

Chapter 4

Heat Transfer

4.1

Introduction

Heat transfer is flow of heat from a high temperature zone to a low temperature zone. Heat is a form of energy and therefore heat transfer rates are represented by units of energy flow per unit time [1, 5, 6]. The symbol used for heat transfer rate is q and the units for heat transfer rate are Btu/hr (USCS) and kW (SI). Note that Watt (W) is the
Js rate of energy transfer and W
Joules s Conversion factor: 1 kW = 3412 Btu/hr

4.1.1

General Equation for Modeling of Heat Transfer

The equation for current flow is: Current Flow =

Potential Difference Resistance

I=

V R

Similarly, the equation for heat transfer can be written in terms of driving force and resistance to heat flow [1, 6]. The driving force for heat transfer is the temperature difference between the hot and cold regions. The resistance to heat flow is known as thermal resistance and it is represented by Rth. Heat Flow =

Temperature Difference Resistance to heat flow

Thus, heat flow is equal to the temperature difference driving force divided by the sum of all the thermal resistances involved in a given situation.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 N. S. Nandagopal, Chemical Engineering Principles and Applications, https://doi.org/10.1007/978-3-031-27879-2_4

269

270

4

q=

Heat Transfer

ΔT Rth

ð4:1Þ

The units for thermal resistance can be obtained by rearranging Eq. 4.1. Rth =

ΔT °F °C

q W Btu=hr

It is more convenient and practical to use the thermal resistance per unit cross section area perpendicular to the heat flow direction and the heat flow per unit cross section area is known as the heat flux. Equation 4.2 is an expression for calculating the heat flux. q = A

4.1.2

ΔT Rth,ua

ð4:2Þ

Heat Transfer Modes

The principal modes of heat transfer are conduction, convection, and radiation [1, 6]. • The transfer of heat through solid media such as a metal pipe wall, building walls, and insulation is called conduction heat transfer. Consider hot fluid flowing in an insulated pipe. The heat will flow through the pipe wall and the covering insulation by means of conduction mechanism. • Convection heat transfer is heat transfer between a solid surface and an adjoining fluid in contact with the solid surface. Consider a flat hot plate exposed to ambient air. Heat is transferred from the hot plate surface to the cooler air by means of convection mechanism. There are two types of convection heat transfer mechanisms, free or natural convection and forced convection [5]. – In free convection, the ambient fluid media remains stationary relative to the solid surface. When a hot plate is exposed to still air, the air particles adjacent to the hot surface get heated up and rise above the surface due to lower density. The rising air particles displace the cooler, heavier air particles farther away from the surface and this cycle continues, setting up “convection current” pathways until all the air particles are sufficiently heated up. Thus, the primary mechanism of heat flow in free convection is the density difference between the hot and cold fluid particles. – In forced convection, the fluid adjacent to the solid surface keeps moving resulting in a faster rate of heat transfer. Consider air being blown across the hot surface of the plate. The heated air particles continue to rise above the surface. However, they are replenished by the advancing colder air particles approaching the plate resulting in faster cooling of the plate surface.

4.2

Conduction Heat Transfer

271

• Unlike conduction and convection, radiation heat transfer does not require any media (solid or fluid) for heat transfer to occur. The radiation heat from sun reaches earth after traveling through vast, empty space (vacuum). Thus, the only requirement for radiation heat transfer is a temperature difference between the hot and cold entities.

4.2

Conduction Heat Transfer

Conduction heat transfer is governed by Fourier’s law of heat conduction [1, 6]. According to Fourier’s law, for one dimensional heat flow, the rate of heat transfer per unit cross section area is proportional to the temperature gradient in the direction of heat flow. Fourier’s law can be mathematically written as, dT q = -k A dx

ð4:3Þ

Since the temperature gradient in the direction of heat flow is always a negative quantity, a negative sign is necessary to keep heat flow as a positive quantity. The variable k in Fourier’s law is called thermal conductivity. Thermal conductivity is a physical property of materials, and it has the following units: k
Btu=hr - ft - ° F ðUSCSÞ

k
W=m:K ðSIÞ

Conversion factor: 1 Btu/hr-ft-°F = 1.731 W/m.K Typically, metals have relatively high thermal conductivities while insulating materials have low thermal conductivities.

4.2.1

Conduction Through a Rectangular Slab

Consider heat flow across a solid slab of thickness ΔX as shown in Fig. 4.1. The heat flow is normal to the cross section area of the slab. Fourier’s law (Eq. 4.3) can be integrated to obtain the following result: X2

q A

T2

dX = - k X1

dT T1

q ΔT =k A ΔX

ð4:4Þ

272

4

Heat Transfer

Fig. 4.1 Fourier’s law of heat conduction

Where, ΔT = T1–T2 and ΔX = X2–X1. Here, the symbol Δ represents the difference between the higher value and the lower value, unlike the representation for change in value in most cases. Equation 4.4 is the integrated form of Fourier’s law for one-dimensional heat flow through a slab [1, 6]. Equation 4.4 can be written in the form of thermal resistance per unit area as shown here. q ΔT ΔT = ΔX =k A ΔX k

ð4:5Þ

Compare Eq. 4.5 with the basic equation for heat flow per unit area (Eq. 4.2, reproduced here for reference). q = A

ΔT Rth,ua

From the comparison of the two equations, the conduction thermal resistance per unit area is Rcond,ua =

ΔX k

ð4:6Þ

Example 4.1 A steel furnace is lined on the inside with a 6-in thick insulating brick. The thermal conductivity of the insulating brick is 0.069 Btu/hr-ft-°F. The temperature on the hot side of the brick is 1500 °F. Determine the following: A. The thermal resistance per unit area of the insulating brick. B. The rate of heat transfer required per unit area of the furnace wall if the temperature of the cooler side of the insulating brick is to be limited to 700 °F.

4.2

Conduction Heat Transfer

273

Solution A. Calculate the thermal resistance per unit area by using Eq. 4.6.

ΔX Rcond,ua = = k

6 in 12 in=1 ft = 7:25 hr - ft2 - ° F=Btu Btu 0:069 hr - ft - ° F

B. Calculate the required heat transfer per unit area by using Eq. 4.2. q ΔT 1500 ° F - 700 ° F = = 110:34 Btu=hr - ft2 = A Rcond,ua hr - ft2 - ° F 7:25 Btu

4.2.1.1

Multilayer Conduction

Conduction through multiple layers can be modeled by adding up the thermal resistances of each layer [1, 6]. Consider heat flow through two solid layers, Layer 1 and Layer 2 as shown in Fig. 4.2. At steady state, the overall heat flow across the multiple layers as well as the heat flow across each layer will be constant and identical in value. The heat transfer rate can be calculated by dividing the overall temperature difference by the sum of the resistances of all layers (Eq. 4.2). The heat transfer rate can also be calculated by dividing the temperature difference of a particular layer by the thermal resistance of the same layer. Once again, it is useful to write the

Fig. 4.2 Conduction through multiple layers

274

4 Heat Transfer

equations in terms of heat transfer per unit cross section area normal to the heat flow direction. ðT - T Þ q ΔT overall = ΔX11 ΔX3 2 = A Rcond,ua k þ k 1

ð4:7Þ

2

q ðΔT Þ1 T 1 - T 2 ðΔT Þ2 T 2 - T 3 = ΔX 1 = = ΔX 2 = R1,ua R2,ua A k1

ð4:8Þ

k2

Example 4.2 A vertical cylindrical furnace has dimensions of 3.5 m OD and 10 m height. The steel shell of the furnace is 60 mm thick, and the thermal conductivity of steel is 45 W/m.K. The temperature of hot gases inside the furnace is 1200 °C and the ambient temperature is 20 °C. The inside surface of the steel shell is shielded by a layer of insulating carbide brick with thermal conductivity 13.7 W/m. °C. The convection and fouling resistances of the hot gases do not have to be considered since they are very small relative to the conduction resistance of the insulating brick. Determine the thickness of the insulating brick required to limit the temperature of the inside surface of steel shell to a maximum value of 300 °C. Solution Due to the large diameter of the cylinder, it can be approximated by a rectangular geometry. Draw the schematic diagram of the multilayer conduction system as shown in the figure. Layer 1 is the insulating brick represented by subscript B and layer 2 is the steel shell represented by subscript S.

4.2

Conduction Heat Transfer

275

Since the convection and fouling resistances do not have to be considered, the temperature of the hot side of the insulating brick will be the same as that of the hot gases, that is, T1 = 1200 °C and the temperature of the cooler side of the steel shell will be the same as that of the ambient air, that is, T3 = 20 °C, as per the nomenclature in the schematic diagram. The temperature of the hotter side of the steel shell should be limited to 300 °C, that is, T2 = 300 °C. At steady state, Heat flow through the brick layer = heat flow through the steel shell This situation is mathematically represented by Eq. 4.8. Therefore substitute all the known values into Eq. 4.8 and solve for the required thickness of the brick layer, ΔX1, as shown. 1200 ° C - 300 ° C q T1 - T2 T2 - T3 300 ° C - 20 ° C = ) = = ) ΔX ΔX ΔX 1m A 1 2 1 60 mm × W k1 k2 1000 mm 13:7 W m  °C 45 m  °C ΔX 1 = 0:0587 m ð58:7 mmÞ

4.2.2

Conduction Through a Cylindrical Wall

The heat transfer through a cylindrical wall occurs in the radial direction [1, 5, 6] as shown in Fig. 4.3. L is the length of the cylinder and T1 > T2 as per the heat flow direction shown in Fig. 4.3. Consider a small element of the cylindrical wall at a distance r from the center and with thickness dr. Fourier’s law of heat conduction can be applied in the radial direction and integrated as shown here.

Fig. 4.3 Conduction through a cylindrical layer

276

4

qr = - kAr

dT dr

Heat Transfer

ð4:9Þ

In Eq. 4.9, Ar, is the surface area of the cylindrical surface perpendicular to the radial heat flow direction and k is the thermal conductivity of the cylindrical wall material. Ar = 2πrL

ð4:10Þ

Combine Eqs. 4.9 and 4.10 and integrate the resulting equation after separating the variables T and r. qr = - k ð2πrLÞ

dT dr T2

r2

dr = - ð2πkLÞ r

qr r1

dT T1

qr = qcyl =

2πkLðT 1 - T 2 Þ ln

r2 r1

ð4:11Þ

Equation 4.11 is the general equation for steady state heat conduction through a cylindrical layer. The most common application of heat conduction through a cylindrical wall is pipes covered with insulation. In process plants, there can be several pipes with similar heat flow conditions but of varying lengths. Hence, it is of practical use to write Eq. 4.11 in terms of heat flow per unit length of the cylinder. qcyl 2πkðT 1 - T 2 Þ = L ln rr21

ð4:12Þ

Equation 4.12 can be written in terms of temperature difference and thermal resistance per unit length (ul) of a cylindrical wall. qcyl ΔT ΔT = = L R r2 cyl,ul ln

ð4:13Þ

r1

2πk

From Eq. 4.13, the thermal resistance per unit length of a cylindrical wall is,

4.2

Conduction Heat Transfer

277

Rcyl,ul =

ln

r2 r1

2πk

ð4:14Þ

Example 4.3 A steel pipe (ID = 5.76 in, OD = 6.63 in, k = 11 Btu/hr-ft-°F) carries steam at 400 °F, which is approximately the temperature of the inside pipe wall. The pipe is covered with a 2-in layer thick fiberglass insulation (k = 0.023 Btu/hr-ft-°F). A. Calculate the thermal resistances per unit length of the pipe wall and the insulation layer and determine the percentage contribution of each to the total thermal resistance per unit length. B. If the temperature of the insulation surface is at 150 °F, calculate the heat loss per foot length of the pipe. Solution A. Calculate the thermal resistances per unit length using Eq. 4.14. Pipe wall: OD 6:63 in = = 3:315 in 2 2 ID 5:76 in = = 2:88 in r1 = 2 2

r2 =

r2 r1 = RPipe, ul = 2πkpipe ln

3:315 in 2:88 in Btu ð2π Þ 0:11 hr - ft - ° F ln

= 0:2035 hr - ft2 - ° F=Btu - ft length Insulation: r 2 = 3:315 in, r 3 = r 2 þ Δr ins = 3:315 in þ 2 in = 5:315 in r3 r2 Rins, ul = = 2πkins ln

5:315 in 3:315 in Btu ð2π Þ 0:023 hr - ft - ° F ln

= 3:267 hr - ft2 - ° F=Btu - ft length Calculate the total thermal resistance per unit length.

278

4

Heat Transfer

Rtotal,ul = Rpipe,ul þ Rins,ul = 0:2035

hr - ft2 - ° F hr - ft2 - ° F þ 3:267 Btu - ft length Btu - ft length

= 3:4705 hr - ft2 - ° F=Btu - ft length % contribution from pipe wall =

0:2035 100 = 5:86% 3:4705

% contribution from insulation =

3:267 100 = 94:14% 3:4705

Comment: The contribution of metal pipe wall to the overall thermal resistance is extremely small and can be safely ignored. This is because metal pipe walls have relatively much higher thermal conductivity compared to insulation materials and thermal resistance is inversely proportional to thermal conductivity. B. Calculate the heat loss per foot length of the pipe by using Eq. 4.13 qcyl ΔT overall = = L Rtotal, ul

4.2.3

400 ° F - 150 ° F = 73:37 Btu=hr - ft hr - ft2 - ° F 3:4075 Btu - ft length

Conduction Through a Spherical Wall

Heat conduction through a spherical shell also occurs in the radial direction as shown in Fig. 4.4 [5, 6]. Consider a cross section of the spherical wall with a small element of thickness dr at a distance r from the center of the sphere. Apply Fourier’s law in the radial direction to the spherical section. For the spherical element, the surface area perpendicular to the heat flow direction is, Ar = 4πr2 From Eqs. 4.9 and 4.15, Fig. 4.4 Conduction through a spherical wall

ð4:15Þ

4.2

Conduction Heat Transfer

279

qr = - k 4πr2

dT dr

ð4:16Þ

Separate the variables in Eq. 4.16 and integrate with appropriate limits. r2

qr 4πk

T2

dr = -k r2 r1

qr 4πk

dT T1

1 1 = ðT 1- T 2 Þ r1 r2

qr = qsphere =

ð4πk ÞðT 1 - T 2 Þ 1 1 r1 r2

ð4:17Þ

Equation 4.17 can be written in terms of temperature difference and thermal resistance of spherical wall. qsphere =

T1 - T2 ΔT = 1 1 R sphere r1 - r2 4πk

ð4:18Þ

From Eq. 4.18, the thermal resistance due to a spherical wall is, 1 1 r1 r2 Rsphere = 4πk

ð4:19Þ

Example 4.4 A spherical tank with ID 178 cm is used in storing liquid ammonia at a temperature of -60°C, which can be considered as the inside surface temperature of the polyurethane insulation (k = 0.028 W/m. K). The average outside surface temperature of the insulation is 15 °C. Determine the thickness of insulation required to limit the heat gain of the spherical tank to 500 W. Solution Calculate the inside radius of the sphere in meters. 1m D1 ð178 cmÞ 100 cm = 0:89 m = r1 = 2 2 Since the heat transfer is from outside the sphere to the inside, T2 > T1. Accordingly, based on Eq. 4.18, the equation applicable in this situation is

280

4

qsphere =

Heat Transfer

ΔT T - T1 = 2 1 1 1 1 r1 r2 r1 r2 4πk 4πk

Substitute the known values into the preceding equation. 500 W =

15 ° C - ð - 60 ° CÞ 1 1 0:89 m r2 W ð4π Þ 0:028 mK

Solve the preceding equation for r2. r 2 = 0:9339 m Calculate the thickness of the insulation required. Δr = r2 - r 1 = 0:9339 m - 0:89 m = 0:044 m ð4:4 cmÞ

4.3

Convection Heat Transfer

Convection heat transfer is heat transfer between a solid surface and a fluid adjacent to the solid surface. The heat transfer mechanism in convection heat transfer involves the motion and circulation of fluid particles. The equation governing convection heat transfer is called Newton’s law of cooling, which states that the heat transfer per unit area of the surface (also known as heat flux) is proportional to the difference between the surface temperature Ts and the temperature of the fluid far away from the surface, T1, also known as the bulk temperature of the fluid [1, 5, 6]. The proportionality constant in Newton’s law of cooling is known as the heat transfer coefficient, h, also known as film coefficient or film conductance. The heat transfer coefficient h has the units Btu/hr-ft2-oF (USCS) and W/m2.oC (SI). Equation 4.20 is the mathematical representation of Newton’s law of cooling. q = hAs ðT s- T 1 Þ In Eq. 4.20, As is the area of the surface in contact with the fluid. Conversion factor: 1 W/m2.oC = 0.17611 Btu/hr-ft2-oF

ð4:20Þ

4.3

Convection Heat Transfer

4.3.1

281

Convection Heat Transfer Resistance

Equation 4.20 can be rearranged to obtain the following equation. q=

Ts - T1 ΔT
1 Rconv hAs

ð4:21Þ

Comparing the terms in the denominator of Eq. 4.21, the resistance to convection heat transfer can be written as shown in Eq. 4.22. Rconv =

1 hAs

ð4:22Þ

Equation 4.21 can be written in terms of heat transfer rate per unit surface area. T - T 1 ΔT conv q = s
1 Rconv,ua As h

ð4:23Þ

Comparing the terms in Eq. 4.23, the resistance per unit surface area to convection heat transfer can be written as shown in Eq. 4.24. Rconv,ua =

1 h

ð4:24Þ

Thus, the resistance to convection heat transfer per unit surface area is the reciprocal of the heat transfer coefficient. Resistance is always inversely proportional to the transfer coefficient. Example 4.5 Air at a bulk temperature of 70 °F flows over a rectangular flat plate maintained at a temperature of 120 °F. The heat transfer coefficient for air is 11.8 Btu/hr-ft2-°F. The length of the plate is 3.7 ft., and the width is 1.4 ft. Calculate the heat transfer from the plate. Solution Calculate the surface area of the plate. As = L × W = 3:7 ft × 1:4 ft = 5:18 ft2 Calculate the heat transfer from the plate using Eq. 4.20.

282

4

q = hAs ðT s - T 1 Þ = 11:8

Btu hr - ft2 - ° F

Heat Transfer

5:18 ft2 ð120 ° F - 70 ° FÞ

= 3056:2 Btu=hr

4.3.2

Dimensionless Parameters Used in Heat Transfer

Calculation of convection heat transfer involves extensive use of heat transfer correlations [1, 5, 6]. The heat transfer correlations typically use several dimensionless parameters. Table 4.1 lists dimensionless parameters used in heat transfer. Example 4.6 Oil flowing in the annular region of a double pipe heat exchanger has the following properties: specific gravity = 0.88, specific heat = 0.61 Btu/lbm-°F, thermal conductivity = 1.128 Btu/hr-ft-°F, and dynamic viscosity = 0.2385 lbm/ft-sec. Determine the Prandtl number of the oil. Solution Use the formula for Prandtl number from Table 4.1 and substitute all the known values with proper care of the units.

Table 4.1 Dimensionless parameters used in heat transfer Dimensionless parameter Reynolds number Grashof number

Symbol Re

Formula Lvρ Lv = μ ν

Physical significance Ratio of inertial force to viscous force

Gr

gL3 βðT s - T 1 Þ ν2

Nusselt number Prandtl number

Nu

hL k cp μ ν = k α

Stanton number Peclet number

St

Rayleigh number

Ra

Ratio of buoyancy force due to density difference to the resisting viscous force in free convection Ratio of heat transferred by convection to heat transferred by conduction Ratio of momentum diffusivity to thermal diffusivity Ratio of heat transfer to the fluid to the heat capacity of the fluid Ratio of heat transfer due to fluid motion to heat transfer due conduction Relative importance of buoyancy forces compared to viscous forces and thermal conduction

Pr

Pe

Nu h = Re Pr ρcp v Lv Re  Pr = α Ra =

gL3 βðT s - T 1 Þ αν

k μ Note: ν = kinematic viscosity = , α = Thermal diffusivity = , c = specific heat (heat capacity) ρcp p ρ of the fluid, L = characteristic dimension (diameter for a pipe, length for a flat plate), β = coefficient of thermal expansion

4.3

Convection Heat Transfer

cp μ Pr = = k

283

Btu lbm 0:2385 lbm - ° F ft - sec Btu 1 hr 1:128 × hr - ft - ° F 3600 sec

0:61

= 464:3

Note: The Prandtl number is purely a function of fluid properties independent of flow parameters such as velocity. As shown in the preceding example, the Prandtl number for oils can be relatively high due to the high value of the viscosity and it can range from 50 to 10,000. The Prandtl numbers for air and gases are typically in the range of 0.6 to 1.2, while for water it is in the range between 1 and 10. The Prandtl numbers for liquid metals are typically very small (10-3 to 10-2) due to high thermal conductivity of liquid metals.

4.3.3

Correlations Used in Calculating Convection Heat Transfer Coefficients

Convection heat transfer coefficients are usually calculated using heat transfer correlations, which are equations typically involving dimensionless parameters. Most of the heat transfer correlations are developed based on results from experimental studies. Hence they are empirical and situation specific (laminar flow through pipes, turbulent flow through pipes, liquid metal heat transfer etc.) [5, 6, 8]. While using heat transfer correlations, it is important to satisfy the constraints associated with the correlations. Correlations for convection heat transfer involve dimensionless parameters outlined in Table 4.1. Dimensionless parameters are calculated using fluid properties. Typically, the fluid properties are evaluated at the average bulk or mixing cup temperature of the fluid. A few important heat transfer correlations for convection heat transfer are presented in Table 4.2. Example 4.7 Hot water, flowing in the annular region (ID of outer pipe, Dio = 6.065 in and OD of inner pipe, Doi = 4.50 in) of a double pipe heat exchanger has the following properties at its average bulk temperature: density = 61.2 lbm/ft3, specific heat = 1.0 Btu/lbm-°F, thermal conductivity = 0.3607 Btu/hr-ft-°F, and dynamic viscosity = 0.000362 lbm/ft-sec. The mass flow rate of hot water is 16 lbm/sec. Calculate the convection heat transfer coefficient for hot water. Solution The annular section is a non-circular section requiring the use of hydraulic diameter. Calculate the hydraulic diameter of the annular region using Eq. 3.30 (Chap. 3, Fluid Mechanics). The area of the annular region is the area between the inner surface of the outer pipe and the outer surface of the inner pipe. The wetted perimeter of the

284

4

Heat Transfer

Table 4.2 Heat transfer correlations for convection heat transfer Situation Laminar flow in pipes

Correlation with name Pr DL

0:065 Re 2 (4.25) 1 þ 0:04 Re Pr DL 3 Dittus–Boelter equation Nu = 0.023Re0.8Prn (4.26) n = 0.4 if fluid is being heated = 0.3 if fluid is being cooled Seider–Tate equation 0:14 μ Nu = 0:027 Re 0:8 Pr1=3 (4.27) μw NuL = 0.664ReL0.5Pr1/3 (4.28) NuL = 0.037ReL0.8Pr1/3 (4.29) Nu = 3:66 þ

Turbulent flow in pipes

Turbulent flow in pipes

Laminar flow over flat plates Turbulent flow over flat plates

Constraints Re < 2100 Re >10,000

Re >10,000

ReL < 5 × 105 5x105 ≤ ReL ≤ 107 0.6 ≤ Pr ≤ 60

annular region is the sum of the circumferences of the outer surface of the inner pipe and the inner surface of the outer pipe. Convert the given diameters to feet.

4:50 in = 0:3750 ft 12 in 1 ft 6:065 in = 0:5054 ft Dio = 12 in 1 ft π Dio 2 - Doi 2 A 4 =4 DH = 4 π ðDio þ Doi Þ P Doi =

= Dio - Doi

= 0:5054 ft - 0:3750 ft = 0:1304 ft Calculate the velocity of water by using the continuity equation (Eq. 3.10). The cross section area of flow is the area of the annual region.

v=

m_ = ρA ρ π 4

m_ Dio 2 - Doi 2

60

= 61:2

lbm ft3

π 4

lbm sec

ð0:5054 ftÞ2 - ð0:3750 ftÞ2

= 10:87 ft=s Calculate the Reynolds number for flow of water in the annular region using Eq. 3.11 and use the hydraulic diameter in place of the diameter, D.

4.3

Convection Heat Transfer

Re =

DH vρ = μ

285

lbm ft 61:2 3 sec ft lbm 0:000362 ft - sec

ð0:1304 ftÞ 10:87

= 239, 635

Calculate the Prandtl number of water using the formula given in Table 4.1.

cp μ Pr = = k

Btu lbm 0:000362 lbm - ° F ft - sec Btu 1 hr 0:3607 × hr - ft - ° F 3600 sec

1:0

= 3:613

For the flow of water through this annular section, since Re = 239,635 > 10,000, calculate the Nusselt number using the Dittus–Boelter equation (Eq. 4.26). The exponent n = 0.3 since water is being cooled. Nu = 0:023 Re 0:8 Pr0:3 = ð0:023Þð239635Þ0:8 ð3:613Þ0:3 = 680:3 Calculate the heat transfer coefficient for water by using the definition of Nusselt number (Table 4.1). The characteristic dimension, L, in the case of a pipe is the hydraulic diameter, DH. Nu = 680:3 =

hDH ) k

Btu 680:3 × k ð680:3Þ 0:3607 hr - ft - ° F = h= 0:1304 ft DH = 1881:8 Btu=hr - ft2 - ° F

4.3.4

Typical Range of Convection Heat Transfer Coefficients

The value of convection heat transfer coefficient depends on the specific situation being considered. For example, heat transfer coefficients for free convection will be much smaller in value compared to heat transfer coefficients for forced convection. Further, in forced convection itself, the value of the heat transfer coefficient depends on the properties of the fluid involved and also the velocity of the fluid. However, typical ranges for convection heat transfer coefficients are observed for commonly encountered situations like flow of air in a duct or flow of water in a pipe/tube. The typical ranges of forced convection heat transfer coefficients [5] for different situations are presented in Table 4.3.

286

4

Table 4.3 Typical ranges for forced convection heat transfer coefficients

Fluid/Situation Air flow Gas/vapor flow Water flow in tubes Liquid flow in tubes Liquid metal flow in pipes Boiling Condensation

Heat Transfer

Range for h (W/m2.K) 10–150 25–250 300–3000 500–5000 3000–30,000 2000–100,000 5000–100,000

Note: To obtain the heat transfer coefficient in Btu/hr-ft2-°F, multiply the values in the table by 0.1761 since 1 W/m2. K = 0.1761 Btu/hr-ft2-°F

4.3.5

Overall Heat Transfer Coefficients in Conduction–Convection Systems

Many practical situations involve a combination of conduction and convection heat transfer [1, 5, 6]. For example, the heat transfer in a building from warm inside air to a cooler outside air involves convection heat transfer from the inside air to the wall, conduction heat transfer through the wall, and finally convection heat transfer between the wall and the outside air. Thus, this situation involves three resistances to heat flow. The overall heat transfer coefficient is the inverse of the overall resistance to heat flow in conduction–convection systems. Consider the conduction–convection system shown in Fig. 4.5. The heat flow is represented by Eq. 4.30. q=

T room - T air ΔT overall = Rconv,i þ Rcond þ Rconv,o Rth

ð4:30Þ

Under steady state conditions, the heat flow is constant through the room air, through the wall, and through the ambient (outside) air. The cross section area of the wall perpendicular to the heat flow direction is also constant. Equation 4.30 can be written in terms of heat flow per unit cross section area and the corresponding resistances (due to conduction and convection) per unit area (Eqs. 4.5 and 4.24). q ΔT overall ΔT overall ΔT overall = = = 1 ΔX 1 Rconv,i,ua þ Rcond,ua þ Rconv,o,ua A Rth,ua þ þ k ho hi

ð4:31Þ

In Eq. 4.31, hi and ho are the convection heat transfer coefficients for the inside and outside air, respectively. The three heat transfer resistances can be combined and

4.3

Convection Heat Transfer

287

Fig. 4.5 Combination of conduction and convection

represented by a single resistance, which is the reciprocal of the overall heat transfer coefficient commonly represented by U. q ΔT overall ΔT overall = = 1 1 ΔX 1 A U þ þ hi k ho

ð4:32Þ

The following equations can be written based on the preceding equation. U=

1 1 ΔX 1 þ þ hi k ho

q = UAΔT overall

ð4:33Þ ð4:34Þ

Overall heat transfer coefficients are extensively used in heat exchanger design equations [2–6]. A heat exchanger facilitates heat transfer from a hot fluid to a cold fluid. In this situation, again there will be three resistances: convection resistance due to the outside fluid, conduction resistance of the tube wall, and convection resistance due to the inside fluid in the tubes. Overall heat transfer coefficients for heat exchangers are discussed in detail later in this chapter. Example 4.8 A metallurgical furnace consists of 2 cm thick insulating ceramic brick (k = 0.48 W/ m.K) on the inside exposed to hot gases at a temperature of 400 °C. The heat transfer coefficient of the hot gases is 57 W/m2.K. Adjacent to the ceramic layer is a 3-cm thick steel shell with relatively negligible conduction resistance. The steel shell is covered with insulation (k = 0.198 W/m.K). The insulation layer is exposed to ambient air at 25 °C with heat transfer coefficient of 33 W/m2.K. Calculate the thickness of the insulating layer required to limit the heat loss per unit area of the furnace wall to a maximum value of 1875 W/m2.

288

4

Heat Transfer

Solution Calculate the overall heat transfer coefficient for this situation using Eq. 4.34. W 1875 2 ðq=AÞ m = = 5:0 W=m2  ° C U= ΔT overall 400 ° C - 25 ° C Neglecting the conduction resistance of the steel shell, the heat transfer resistances involved in this situation are: inside convection resistance of hot gases, conduction resistance of ceramic layer, conduction resistance of insulation, and outside convection resistance of ambient air. Subscript “c” represents the ceramic layer and Subscript “i” represents the insulation. For this situation, Eq. 4.33 can be written as follows and then all the known values can be substituted. 1 1 ΔX ΔX 1 = þ cþ iþ U hi ho kc ki 1m 2 cm × ΔX i 1 1 1 100 cm þ = þ þ W W W W W 57 2 0:48 5 2 0:198 33 2 mK mK m K m K m K Solve the preceding equation for ΔXi, the thickness of insulation required. ΔX i = 0:0219 m ð2:19 cm ≃ 22 mmÞ

4.3.6

Order of Magnitude Analysis to Determine the Value of Overall Heat Transfer Coefficients

In conduction–convection systems, the value of the overall heat transfer coefficient will be close to the reciprocal value of the controlling resistance [5, 6]. This concept is illustrated in the following example involving a building wall. Consider a facility where, Inside heat transfer coefficient, hi = 40 W/m2.K Thickness of board, ΔX = 1.5 cm and thermal conductivity of board material, k = 0.82 W/m.K Outside heat transfer coefficient, ho = 4 W/m2.K Calculate the individual resistances per unit area.

4.3

Convection Heat Transfer

289

Inside convection resistance per unit area: Ri =

1 = hi

1 = 0:025 m2  K=W W 40 2 m K

Board conduction resistance per unit area: 1m ΔX 1:5 cm × 100 cm Rw = = 0:0183 m2  K=W = W k 0:82 mK Outside convection resistance per unit area: Ro =

1 = ho

1 = 0:25 m2  K=W W 4 2 m K

Calculate the total resistance and then the percentage contribution of each resistance to the total resistance. Rtotal = Ri þ Rw þ Ro = 0:025

m2  K m2  K m2  K þ 0:0183 þ 0:25 W W W

= 0:2933 m2  K=W

The percentage contribution of inside convection resistance is,

Ri × 100 = Rtotal

m2  K W m2  K 0:2933 W 0:025

× 100 = 8:52%

The percentage contribution of wall conduction resistance is,

Rw × 100 = Rtotal

m2  K W m2  K 0:2933 W 0:0183

× 100 = 6:24%

The percentage contribution of outside convection resistance is,

Ro × 100 = Rtotal

m2  K W m2  K 0:2933 W 0:25

× 100 = 85:24%

290

4

Heat Transfer

Thus, the outside convection resistance is the controlling resistance with a contribution of approximately 85% to the total or overall resistance. In this example, the overall heat transfer coefficient is U=

1 = Rtotal

1 = 3:41 W=m2  K m2  K 0:2933 W

Thus, the value of the overall heat transfer coefficient (3.41 W/m2.K) is close to the value of the outside heat transfer coefficient (4 W/m2.K). In any situation, if the order of magnitude of one of the heat transfer coefficients is significantly lower compared to the value of the other heat transfer coefficient, then the overall heat transfer coefficient will be approximately equal to the significantly lower heat transfer coefficient.

4.4

Heat Exchangers

Heat exchangers [2–6] facilitate heat transfer from a hot fluid to a cold fluid. The two most widely used heat exchangers are: shell and tube heat exchangers [3, 4], and double pipe heat exchangers [3, 4]. The flow arrangement can be either parallel (both fluids have the same flow direction) or counterflow (the two fluids flow in opposite directions). The counterflow arrangement is usually preferred since it results in a greater average temperature difference between the hot and cold fluids. Figures 4.6 and 4.7 illustrate the typical layouts of shell and tube, and double pipe heat exchangers.

Fig. 4.6 Shell and tube heat exchanger schematic. (Source: https://commons.wikimedia.org/wiki/ File:Shell_heat_exchanger_LS.JPG,Public Domain)

4.4

Heat Exchangers

291

Fig. 4.7 Schematic of a counterflow double pipe heat exchanger

4.4.1

Heat Balance

The heat balance [5, 6] in a heat exchanger is an application of conservation of energy principle. Ideally, the exchanger is well insulated, resulting in negligible heat transfer to/from the surroundings. With this reasonable assumption, all the energy released by the hot fluid (subscript “h”) is absorbed by the cold fluid (subscript “c”). The rate of heat exchange between the hot and cold fluids is also known as the “heat duty” of the exchanger represented by the symbol q. The heat balance equation is: q = m_ h cph ΔT h = m_ c cpc ΔT c

ð4:35Þ

The heat balance equation is particularly important in solving heat exchanger problems. Standard values of specific heat of water, cp = 1.0 Btu/lbm-°F (USCS) and cp = 4.187 kJ/kg.K (SI)

4.4.2

Log Mean Temperature Difference (LMTD)

The driving force for heat transfer in a heat exchanger is the temperature difference between the hot fluid and the cold fluid. This temperature difference varies along the length of the exchanger as shown in Figs. 4.8a and 4.8b. The log mean temperature difference (LMTD) represents an average temperature difference between the hot and cold fluids in the exchanger [2–6]. The log mean temperature difference can be calculated by using the following formula written in terms of temperature differences at end “1” and at end “2” of the heat exchanger. ΔT LMTD =

ΔT 1 - ΔT 2 ΔT 2 - ΔT 1 = ΔT 1 ΔT 2 ln ln ΔT 2 ΔT 1

ð4:36Þ

292

4

Heat Transfer

Fig. 4.8a Variation in temperature difference for parallel flow

Fig. 4.8b Variation in temperature difference for counterflow

4.4.3

Overall Heat Transfer Coefficient for Heat Exchangers

The overall heat transfer coefficient [2, 3, 5, 6] used in the design of heat exchangers is the reciprocal of the overall resistance to heat transfer. In the most general case, there are five resistances to heat transfer between the hot and cold fluids in a heat exchanger. Proceeding from the fluid inside the tubes to the inner surface of the outside pipe, the resistances are as follows: 1. Convection resistance due to the fluid inside the tube = 2. Fouling resistance due to the inside fluid = Ri/Ai

1 hi Ai

lnðro =r i Þ lnðDo =Di Þ = 2πkL 2πkL 4. Fouling resistance due to the fluid outside the tube = Ro/Ao 1 5. Convection resistance due to the outside fluid = ho Ao

3. Conduction resistance due to the inside pipe wall =

The fouling factors, Ri, and Ro, have the reciprocal units of heat transfer coefficient (hr-ft2-oF/Btu or m2.oC/W). Fouling resistances are due to the formation of coatings on tube surfaces. The heat transfer areas are the inside and outside surface areas of the tube (not the cross section area). Therefore, Ao = πDoL and Ai = πDiL,

4.4

Heat Exchangers

293

the outside and inside surface areas of the inner tube. L is the length of the exchanger and Di and D0 are the ID and OD of the inner pipe. The difference between the inside and outside surface areas of the tube gives rise to two overall heat transfer coefficients. Ui is the overall heat transfer coefficient based on the inside surface area, Ai, of the inner tube and Uo is the overall heat transfer coefficient based on the outside surface area, Ao, of the inner tube. The overall resistance is the sum of the individual resistances. Therefore, lnðDo =Di Þ Ro 1 1 R 1 1 þ þ = = þ iþ 2πkL U o Ao hi Ai Ai Ao ho Ao U i Ai

ð4:37Þ

Since Ai = πDiL and Ao = πDoL, the overall heat transfer coefficient, Ui, can be calculated by using the following equation obtained by multiplying Eq. 4.37 throughout by Ai = πDiL. ðD Þ lnðDo =Di Þ 1 1 Di Di 1 = þ Ri þ i þ þ Ro U i hi ho D o Do 2k

ð4:38Þ

Similarly, the overall heat transfer coefficient, Uo, can be calculated by using the following equation obtained by multiplying Eq. 4.37 throughout by Ao = πDoL. 1 1 = U o hi

Do Di

þ Ri

Do Di

þ

ðDo Þ lnðDo =Di Þ 1 þ Ro þ 2k ho

ð4:39Þ

Invariably, the conduction resistance due to the tube wall can be neglected because the thermal conductivity of the tube wall material (usually copper or steel) is high, and the wall resistance will be much smaller than the other resistances. Further, by assuming Di ≈ Do, the equation for the overall heat transfer coefficient takes the following simple form. 1 1 1 1 1 = = = þ þ Ri þ Ro U o U i U hi ho

ð4:40Þ

Sometimes, the inside and outside fouling factors are combined into a single fouling factor, Rf. Following conservative design practices, heat exchangers are designed using a design overall heat transfer coefficient, UD. The overall heat transfer coefficient under clean conditions, UC, does not include the fouling factor. Equation 4.40 can be simplified using the preceding concepts. 1 1 1 = þ þ Rf hi ho UD

ð4:41Þ

1 1 1 = þ hi ho UC

ð4:42Þ

Since the reciprocal of UD includes an additional resistance due to fouling, UC > UD. Therefore, heat exchangers will be overdesigned when they are first put

294

4

Table 4.4 Typical fouling resistances for different fluids

Heat Transfer

Fouling resistance, hr‐ft2 ‐ ° F Rf Btu

Fluid Cooling water Organic liquids Refrigerants Salt solutions Steam Organic vapors Flue gases

0.0014 0.0011 0.0014 0.0028 0.0009 0.0011 0.0019

Note: Multiply the values listed in the table by 0.1762 to obtain the values in m2. K/W

into service, and they will have more surface area than required. Combining Eqs. 4.41 and 4.42 results in the following relationship between the design overall heat transfer coefficient, UD, and the overall heat transfer coefficient under clean conditions, UC. 1 1 = þ Rf UD UC

ð4:43Þ

Equation 4.43 is frequently used in the design of heat exchangers. Table 4.4 lists typical fouling resistances [5, 6] for different fluids. Conversion factor for fouling resistance: ∘

∘

1:0 hr - ft2 - F=Btu = 0:1762 m2 : C=W Table 4.5 lists typical ranges of design overall heat transfer coefficients for different pairs of fluids with fouling included [5]. Conversion factor: 1.0 Btu/hr - ft2 - °F = 5.678 W/m2 - °C

4.4.4

Heat Exchanger Design Equation

The heat exchanger design equation [2–4, 6] is used in determining the heat transfer surface area, As, required for the exchanger. The general form of the heat exchanger design equation is q = UAs FΔT LMTD

ð4:44Þ

4.4

Heat Exchangers

Table 4.5 Typical overall heat transfer coefficients (U ) for shell and tube heat exchangers U (Btu/hr-ft2-°F)

295 Fluid combination Water – water Organic liquid – organic liquid Heavy oil – heavy oil Light oil – light oil Gas – gas Gas – water Organic liquid – water Organic vapor – water Steam – organic liquid Steam – water Light oil – steam/water Heavy oil – steam/water Steam – brine

Design U range 150–250 30–90 10–50 15–60 5–10 5–50 40–120 125–150 100–200 150–250 50–150 10–60 250,500

Note: Multiply the values listed in the table by 5.678 to obtain the values in W/m2. K

The heat duty, q, is determined from the heat balance equation (Eq. 4.35) and the log mean temperature difference is determined from Eq. 4.36. In Eq. 4.44, F is the MTD correction factor and it is used only for multi-pass heat exchangers, and it can be determined from the graphs (Figs. 4.9a and 4.9b). The MTD correction factor is 1 for double pipe heat exchangers and shell and tube exchangers with a single-pass and also for multi-pass condensers and evaporators. Example 4.9 5300 lbm/hr. of hot oil (specific heat = 0.62 Btu/lbm-°F) at 180 °F enters the tube (ID = 12 in) of a double pipe heat exchanger. Cooling water available at 60 °F flows in the outer tube and the temperature rise of the water is to be limited to 20 °F. The heat duty of the exchanger is estimated to be 3.53 × 105 Btu/hr. The design overall heat transfer coefficient is 155 Btu/hr - ft2 - °F. Determine A. B. C. D. E.

the exit temperatures of oil and water. the mass flow rate of cooling water required. the log-mean temperature difference. the total heat exchanger surface area required. the required length of the 12-in tube.

Solution Subscript “o” is used for oil, the hot fluid, and subscript “w” is used for water, the cold fluid.

296

4

Heat Transfer

Fig. 4.9a MTD correction factors for one shell pass and multiple tube passes (2, 4, 6,. . .). (Source: Basic Design Methods of Heat Exchangers (by Cuneyt Ezgi), Open Access Peer Reviewed Chapter, https://www.intechopen.com/chapters/54521)

Fig. 4.9b MTD correction factors for two hell passes and multiple tube passes (4, 8, 16,. . .). (Source: Basic Design Methods of Heat Exchangers (by Cuneyt Ezgi), Open Access Peer Reviewed Chapter, https://www.intechopen.com/chapters/54521)

A. Calculate the exit temperature of oil by using heat balance equation, Eq. 4.35.

q ΔT o = = m_ o cp,o

3:53 × 105

Btu hr

= 107 ° F lbm Btu 0:62 hr lbm - ° F T exit,o = T inlet,o - ΔT o = 180 ° F - 107 ° F = 73 ° F 5300

Calculate the exit temperature of water using its inlet temperature and the maximum permissible temperature rise.

4.4

Heat Exchangers

297

T exit,w = T inlet,w þ ΔT w = 60 ° F þ 20 ° F = 80 ° F B. Calculate the mass flow rate of water by using heat balance equation, Eq. 4.35.

q = m_ w = cp,w ΔT w

3:53 × 105 1:0

Btu hr

Btu ð20 ° FÞ lbm - ° F

= 17650 lbm=hr

C. Calculate the log mean temperature difference by using the temperature schematic shown below and Eq. 4.36. 1 60 ° F 73 ° F ΔT 1 = 13 ° F ΔT LMTD =

Water Oil

2 ! 80 ° F 180 ° F ΔT 2 = 100 ° F

ΔT 2 - ΔT 1 100 ° F - 13 ° F = 42:64 ° F = 100 ° F ΔT 2 ln ln 13 ° F ΔT 1

D. Rearrange the heat exchanger design equation (Eq. 4.44) to obtain a formula for the heat exchanger surface area required. Substitute all the known values, including 1.0 for the MTD correction factor, F, for double pipe heat exchangers and calculate the heat exchanger surface area required.

q = As = UFΔT LMTD

3:53 × 105

Btu hr

Btu 155 ð1:0Þð42:64 ° FÞ hr - ft2 - ° F

= 53:41 ft2

E. Calculate the length of each tube by using the heat exchanger surface area required, As. L=

As = πDi

53:41 ft2 1 ft ðπ Þ 12 in × 12 in

= 17 ft

Example 4.10 An evaporator with a heat transfer surface area of 120 ft2 is used in evaporating a process liquid with a normal boiling point of 155 °F. The overall heat transfer

298

4 Heat Transfer

coefficient is 625 Btu/hr-ft2-°F. Condensing steam at 15.3 psig is used as the heating medium. Determine the mass flow rate of steam required. Solution Subscript “s” is used for steam, and subscript “p” is used for the process liquid. Calculate the absolute pressure of steam. Pabs = Pgage þ Patm = 15:3 psig þ 14:7 psi = 30 psia Determine the saturation temperature and enthalpy of condensation of steam at 30 psia from steam tables or from any online steam properties resource. T s = T sat ð30 psiaÞ = 250 ° F, hfg = 945 Btu=lbm Source: Spirax Sarco Steam Tables, https://www.spiraxsarco.com/resources-anddesign-tools/steam-tables/saturated-water-line The temperature of the vaporizing process liquid will be constant at 155 °F, and the temperature of the condensing steam will be constant at 250 °F. Therefore there is a constant temperature difference between the hot and cold fluids in the evaporator. ΔT = T s - T p = 250 ° F - 155 ° F = 95 ° F Calculate the heat duty of the evaporator using the heat exchanger design equation (Eq. 4.44), modified slightly for this application. q = UAs ΔT = 625

Btu hr - ft2 - ° F

120 ft2 ð95 ° FÞ = 7125000 Btu=hr

From heat balance, Heat duty of evaporator = heat supplied by condensing steam Therefore, Btu 7125000 q hr = 7540 lbm=hr = q = m_ s hfg ) m_ s = Btu hfg 945 lbm

4.4

Heat Exchangers

4.4.5

299

LMTD Correction Factors

In multi-pass heat exchangers, it is common for the tube side fluid to make several passes through the exchanger. In a “1–2 Exchanger,” the shell side fluid makes a single pass through the exchanger and the tube side fluid makes two passes through the exchanger. Hence, the nomenclature is “1–2 Exchanger.” In a 1–2 exchanger, there is parallel flow during the first tube pass and counterflow during the second tube pass. Usually, LMTD is calculated based on counterflow since it results in a higher value. However, in multi-pass exchangers, the LMTD value will be lower than the calculated value due to the drop in LMTD during parallel flow. The LMTD calculated based on counterflow is adjusted by using a correction factor [4, 6] known as “MTD correction factor.” MTD correction factors are obtained from graphs generated by TEMA (Figs. 4.9a and 4.9b). The MTD correction factor will be 1.0 for single pass (1–1) exchangers, and for condensers and evaporators. This is because condensation and evaporation are isothermal (constant temperature) processes resulting in phase change. Example 4.11 A 1–2 shell and tube heat exchanger is used in cooling a process liquid (specific heat = 0.90 Btu/lbm-°F) from 205 °F to 125 °F using cooling water on the tube side. The cooling water enters the exchanger at 70 °F and leaves the exchanger at 90 °F. The mass flow rate of the process liquid is 26.71 lbm/min. The heat exchanger surface area is 24 ft2. Determine the overall heat transfer coefficient that was used in the design of the exchanger. Solution Subscript “p” represents the process liquid and subscript “w” represents cooling water. Calculate the heat duty of the exchanger using the heat balance equation (Eq. 4.35) for the process liquid. q = m_ p cp, p ΔT p =

26:71

lbm 60 min × min hr

0:9

Btu ð205 ° F - 125 ° FÞ lbm - ° F

= 115,387 Btu=hr Calculate the log mean temperature difference by using the temperature schematic shown below and Eq. 4.36. 1 2 Process Liquid 205 ° F ! 125 ° F Water 70 ° F 90 ° F ΔT 1 = 115 ° F ΔT 2 = 55 ° F

300

4

ΔT LMTD =

Heat Transfer

ΔT 1 - ΔT 2 115 ° F - 55 ° F = = 81:34 ° F 115 ° F ΔT 1 ln ln ΔT 2 55 ° F

Since this is a 1–2 shell and tube exchanger, the MTD correction factor, F, must be used. F is determined by using Fig. 4.9a. Calculate the parameters P and R as per the nomenclature in Fig. 4.9a. P=

t2 - t1 90 ° F - 70 ° F = = 0:15 T 1 - t 1 205 ° F - 70 ° F

R=

T 1 - T 2 205 ° F - 125 ° F = = 4:0 t2 - t1 90 ° F - 70 ° F

Using P and R as parameters, determine the MTD correction factor from the graph as shown.

From the graph, the MTD correlation factor, F = 0.95. Rearrange the heat exchanger design equation (Eq. 4.44) to obtain a formula for the design heat transfer coefficient. Substitute all the known values and calculate the design heat transfer coefficient. Btu 115387 q hr = = 62:22 Btu=hr - ft2 - ° F U= As FΔT LMTD 24 ft2 ð0:95Þð81:34 ° FÞ

4.4.6

Heat Exchanger Effectiveness

Heat exchanger effectiveness, ε, is defined as the ratio of actual heat transfer to the maximum possible heat transfer [4–6].

4.4

Heat Exchangers

301

ε=

qactual qmax

ð4:45Þ

The product of the mass flow rate and heat capacity of a fluid is known as the “heat capacity rate” and is represented by the letter C. The heat capacity rates of the hot and cold fluids can be written in terms of the following equations. Ch = m_ h cph and C c = m_ c cpc

ð4:46Þ

The lower of Ch and Cc values is designated as Cmin and the higher value is designated as Cmax. The maximum possible temperature difference is Th,in - Tc,in and therefore the maximum possible heat transfer can be calculated by using the following equation: qmax = C min ðT h,in - T c,in Þ

ð4:47Þ

In terms of the heat capacity rates, the actual heat transfer is qactual = Cc ΔT c = C h ΔT h

ð4:48Þ

Using Eqs. 4.44, 4.46, and 4.47, the heat exchanger effectiveness can be written as: ε=

C ΔT ðor Ch ΔT h Þ qactual = c c qmax Cmin ðT h,in - T c,in Þ

ð4:49Þ

Knowing the effectiveness of the heat exchanger, the actual heat transfer can be calculated by the following equation. qactual = C c ΔT c = Ch ΔT h = εCmin ðT h,in - T c,in Þ

4.4.7

ð4:50Þ

Effectiveness–NTU Method

During the operation of a process facility, sometimes it becomes necessary to change the flow rate of the process fluids. In heat exchangers, changing the flow rate of one of the fluids creates a situation where the exit temperatures of both the fluids become unknown. In such cases, a trial and error procedure will have to be used. The trial and procedure consists of assuming the exit temperature of one of the fluids and checking if the assumed temperature satisfies the heat balance and heat exchanger design equations. However, such problems can be solved quickly and explicitly by using the Effectiveness-NTU method [4–6], without the need to resort to trial and error procedures. The Effectiveness-NTU method is explained here.

302

4

Heat Transfer

The number of transfer units (NTU) required for heat transfer depends on the overall heat transfer coefficient, the heat exchanger surface area, and the heat capacity rate. NTU is a dimensionless parameter and is calculated using the following equation. NTU =

UAs Cmin

ð4:51Þ

The ratio of Cmin to Cmax is defined as the heat capacity rate ratio, Cr. Cr =

C min Cmax

ð4:52Þ

The heat exchanger effectiveness can be obtained from formulas or from graphs. The formulas are given in terms of NTU and Cr. Double pipe heat exchanger (parallel flow): ε=

1 - exp½ - NTUð1 þ Cr Þ 1 þ Cr

ð4:53Þ

Double pipe heat exchanger (counterflow): ε=

1 - exp½ - NTUð1 - Cr Þ 1 - C r exp½ - NTUð1 - Cr Þ

ð4:54Þ

Shell and tube exchangers:

ε = 2 1 þ Cr þ 1 þ Cr

2 1=2

1 þ exp - NTU 1 þ C r 2

1=2

1 - exp - NTU 1 þ C r 2

1=2

-1

ð4:55Þ

Heat exchanger effectiveness can also be determined from graphs, where the heat exchanger effectiveness is plotted vs. NTU with Cr as the parameter. Figs. 4.10a, 4.10b, and 4.10c are plots of heat exchanger effectiveness for different heat exchanger configurations, that is 1–1, 1–2, 2–4, and so on. Example 4.12 Hot oil (specific heat = 3.5 kJ/kg. °C) flows over the tubes of a 1–1 counterflow shell and tube heat exchanger at a mass flow rate of 3700 kg/hr. Water (specific heat = 4.187 kJ/kg. °C) circulates through the tubes of the exchanger at a rate of 5900 kg/hr. The temperature of the cooling water entering the tubes is 15 °C and oil enters the shell at 95 °C. The overall heat transfer coefficient can be taken as 575 W/m2.K. A heat exchanger with a heat transfer surface area of 25 m2 is used. Determine the exit temperatures of oil and water under the specified conditions.

4.4

Heat Exchangers

303

Fig. 4.10a Heat exchanger effectiveness for 1–1 counterflow heat exchanger. (Source: Basic Design Methods of Heat Exchangers (by Cuneyt Ezgi), Open Access Peer Reviewed Chapter, https:// www.intechopen.com/ chapters/54521)

Fig. 4.10b Heat exchanger effectiveness for shell and tube heat exchanger (1 shell pass and 2, 4, 6,. . .tube passes). (Source: Basic Design Methods of Heat Exchangers (by Cuneyt Ezgi), Open Access Peer Reviewed Chapter, https:// www.intechopen.com/ chapters/54521)

Solution Calculate the heat capacity rates of the hot fluid (oil) and the cold fluid (water) using Eq. 4.46. C h = m_ h cph = 3700 Cc = m_ c cpc = 5900

kg hr

kg hr

3:5 4:187

kJ = 12950 kJ=hr  ° C kg ° C kJ = 24703 kJ=hr  ° C kg  ° C

Since Cc > Ch, Cc = Cmax = 24,703 kJ/hr.°C and Ch = Cmin = 12,950 kJ/hr. °C.

304

4

Heat Transfer

Fig. 4.10c Heat exchanger effectiveness for shell and tube heat exchanger (2 shell passes and 4, 8, 12. . .tube passes). (Source: Basic Design Methods of Heat Exchangers (by Cuneyt Ezgi), Open Access Peer Reviewed Chapter, https:// www.intechopen.com/ chapters/54521)

Calculate the number of transfer units (NTU) using Eq. 4.51 UAs NTU = = C min

W ð25 m2 Þ  °C = 3:996 ≃ 4:0 kJ 1000 J 1 hr 12950 × × hr  ° C kJ 3600 s 575

m2

Calculate the heat capacity rate ratio, Cr, using Eq. 4.52. kJ 12950 Cmin hr  ° C = 0:5242 Cr = = kJ C max 24703 hr  ° C Using NTU and Cr as parameters, determine the heat exchanger effectiveness using Fig. 4.10a for a 1–1 exchanger, as shown in the figure.

4.5

Radiation

305

From the graph, the heat exchanger effectiveness is ε = 92 % (
0.92). Calculate the temperature increase of water using Eq. 4.50. qactual = C c ΔT c = εC min ðT h,in - T c,in Þ ) εC ðT - T c,in Þ ΔT c = min h,in Cc kJ ð95 ° C - 15 ° CÞ ð0:92Þ 12950 hr  ° C = 73:6 ° C = kJ 12950 hr  ° C Calculate the exit temperature of water. T c,out = T c,in þ ΔT c = 15 ° C þ 73:6 ° C = 88:6 ° C Calculate the temperature decrease of oil using Eq. 4.50. qactual = C h ΔT h = εC min ðT h,in - T c,in Þ ) εC ðT - T c,in Þ ΔT h = min h,in Ch kJ ð95 ° C - 15 ° CÞ ð0:92Þ 12950 hr  ° C = = 38:6 ° C kJ 24703 hr  ° C Calculate the exit temperature of oil. T h,out = T h,in - ΔT h = 95 ° C - 38:6 ° C = 56:4 ° C

4.5

Radiation

Every object emits electromagnetic radiation [1, 6] that depends on the absolute temperature (K or 0R) of the object. The radiation energy emitted by a body is due to the vibrational and rotational motions of molecules, atoms, and electrons within the body. The flow of heat caused by the emission of radiant energy is called Radiation heat transfer. The vibrational and rotational activities of the fundamental particles in a body are directly related to the temperature of the body. Therefore, the rate of radiation energy (heat) transfer from a body increases with increasing temperature. In reality, there is a constant exchange of radiant energy between bodies. Consider a red hot metal rod in a room. The rod emits considerable amount of radiant energy due to its high temperature and this energy reaches the surrounding objects such as the walls of the room. However, the walls of the room also emit radiation energy by

306

4 Heat Transfer

virtue of the fact that the walls are at a temperature higher than absolute zero. Some of the radiation energy from the walls could reach the red hot metal rod depending on the geometry, configuration, and positioning of the walls and the rod. Thus, radiation heat transfer involves net energy exchange between the hot and cold bodies [1, 6]. Since the temperature of the red hot metal rod (designated as body “1”) is much higher than the temperature of the surrounding walls (designated as body “2”), there will be a net radiation heat flow from the red hot metal rod to the walls. The net heat transfer from body “1” to body “2” is symbolically represented as q12. The calculation of the net radiation heat transfer is described in detail later.

4.5.1

Stefan–Boltzmann’s Law of Thermal Radiation

Stefan–Boltzmann’s law quantifies the radiation heat emitted by a body. It states that the radiation energy emitted per unit surface area of a perfect or ideal radiation emitting body (also known as a “black body”), is proportional to the fourth power of absolute temperature [1, 6]. An ideal radiator or black body is one that can emit the maximum possible radiation by virtue of its absolute temperature. A black body also absorbs all the radiation that is incident upon it. Black bodies are represented with subscript “b” in equations. Mathematically, Stefan–Boltzmann’s law can be written as Eb / T 4

ð4:56Þ

In Eq. 4.56, Eb is energy emitted by a black body per unit surface area per unit time, which is the same as the heat flux due to radiation with units W/m2 (SI) and Btu/hr-ft2 (USCS). Also in Eq. 4.56, T is the absolute temperature of the body in Kelvin (SI) or degree Rankine (USCS), which can be obtained from the following equations: K = ° C þ 273 and ° R = ° F þ 460 The proportionality relationship in Eq. 4.56 can be transformed into a regular equation by using a proportionality constant known as Stefan–Boltzmann’s constant, represented by the symbol σ. The values of Stefan–Boltzmann’s constant in both the SI and USCS units are given here. σ = 5:669 × 10 - 8 W=m2  K4 ðSIÞ σ = 0:1714 × 10 - 8 Btu=hr - ft2 - ° R4 ðUSCSÞ Based on Stefan–Boltzmann’s law, the rate of radiation heat flux (heat transfer per unit surface area) from a black body can be calculated by using Eq. 4.57.

4.5

Radiation

307

E b = σT 4

4.5.2

ð4:57Þ

Non-Ideal Radiators – Gray Bodies

In most practical situations, objects emit only a fraction of the radiation energy that is predicted by Stefan–Boltzmann’s law. This is because the objects are not perfect radiators and do not emit the same amount of energy as a black body. Thus, most of the objects are gray bodies. Gray bodies are characterized by a factor known as emissivity represented by the symbol, ε. The emissivity of a gray body is defined as the ratio of the actual energy, E, emitted by the body to the energy emitted by a black body, Eb, at the same temperature [1, 6]. Mathematically, emissivity is represented as shown in equation in Eq. 4.58. ε=

E Eb

ð4:58Þ

The radiation heat transfer per unit surface area of a gray body can be calculated by combining Eqs. 4.57 and 4.58. E = εE b = εσT 4

ð4:59Þ

The emissivity of materials depends on the nature of the surface. Polished surfaces tend to have low values of emissivity since they reflect most of the incident radiation. Rough and weathered surfaces tend to have relatively higher values of emissivity. Typical values of emissivity of gray surfaces are εsteel = 0:80, εbrick = 0:88, εwater = 0:88, εplastic = 0:88

4.5.3

Absorptivity, Transmissivity, and Reflectivity

The incident radiation on a surface (also known as irradiation), can be either absorbed or transmitted or reflected. The fraction of irradiation absorbed is called absorptivity, α, the fraction of irradiation transmitted is known as transmissivity, τ, and the fraction of irradiation reflected is called reflectivity, ρ. The sum of these fractions should be one. α þ τ þ ρ=1

ð4:60Þ

Kirchhoff’s law of radiation [1, 6] states that objects have identical capacities to emit as well as absorb radiation. For a black body, the absorptivity is equal to emissivity, and both are equal to unity since a black body absorbs all the irradiation and also emits the maximum possible radiation at a given temperature. For a black body,

308

4

α=ε=1

4.5.4

Heat Transfer

ð4:61Þ

Radiation View Factor

Radiation is due to electromagnetic waves emanating from a surface. Hence, it is a phenomenon based on the characteristics of the surface. As explained earlier, radiation heat transfer involves net heat exchange between two bodies, body “1” and body “2.” Since radiation heat transfer is a surface-based phenomenon, the net radiation heat transfer from body “1” to body “2” (represented as q12 and assuming T1 > T2) will depend on the geometric configuration of the two surfaces. To include the effects of geometric configuration in the calculation of net radiation heat transfer from body “1” to body “2,” a factor known as radiation view factor is used [1, 5, 6]. The radiation view factor is represented by the symbol, F12 and it is also known as shape factor or configuration factor. Depending on the geometric configuration of the two bodies, it is likely that some radiation from body “1” will bypass body “2.” The view factor, F12, represents the fraction of total radiation from body “1” that actually reaches body “2.”

4.5.5

View Factor Relationships

The view factor for net radiation heat flow from body i to body j can be represented by Fij. If Ai represents the surface area of body i, it can be shown that the following view factor relationships (also known as ‘shape factor algebra’) are valid in radiation heat transfer. Ai F ij = Aj F ji

ð4:62Þ

n

F ij = 1

ð4:63Þ

j=1

Equation 4.62 is known as the Reciprocity Rule and Eq. 4.63 is known as the Summation Rule.

4.5.6

Calculation of Net Radiation Heat Transfer

Consider two objects, “1” and “2” that are exchanging heat by radiation with the following assumptions:

4.5

Radiation

309

1. Both the objects are gray bodies, with emissivities ε1 and ε2. 2. The temperature of object “1” is greater than the temperature of object “2.” The net radiation heat transfer from body 1 to body 2 can be conveniently modeled using the concept of driving force and resistances [1, 6]. The driving force is based on the temperature difference between bodies 1 and 2. Radiation heat transfer driving force = Eb1 - Eb2 = σ(T14 - T24) The net radiation heat transfer from body 1 to body 2 encounters the following resistances: Surface resistance of body 1 =

1 - ε1 ε1 A1

Space resistance between bodies 1 and 2 = Surface resistance of body 2 =

1 A1 F 12

1 - ε2 ε2 A2

Based on the concept of heat flow being equal to the driving force divided by the sum of resistances, the equation for net radiation heat transfer from body 1 to body 2 can be written by combining the preceding equations. q12 =

σ T 14 - T 24 1 1 - ε2 1 - ε1 þ þ A1 F 12 ε1 A 1 ε2 A2

ð4:64Þ

Equation 4.64 is a general equation for net radiation heat transfer from body 1 to body 2 and it can be simplified for specific cases.

4.5.7

Radiation Heat Transfer from a Small Object to an Enclosure, Both Being Gray Bodies

All the radiation from the small object (body 1) will reach the enclosure (body 2). Therefore, F12 = 1.0. Also the surface area of the enclosure is much larger than the surface area of the small object. Therefore, A2 > > > > > > A1 and Substitute the preceding results into Eq. 4.64.

1 - ε2 =0 ε2 A 2

310

4

q12 =

Heat Transfer

σ T 14 - T 24 = σε1 A1 T 1 4- T 2 4 1 1 - ε1 þ A1 ε1 A1

ð4:65Þ

Hint for easy calculation in situations involving radiation: Absolute temperatures to the fourth power typically have very high numerical values. Calculations related to radiation heat transfer can avoid the problem of dealing with very high value numbers by dividing the absolute temperature by 100 and then taking the fourth power, which will result in the handling of reasonably smaller numbers. This approach works because dividing the absolute temperature by 100 should be accompanied by multiplying the absolute temperature by 100. Raising 100 to the fourth power will result in 108 which will cancel with 10-8 in the Stefan–Boltzmann’s constant. Example 4.13 Two black parallel plates face each other as shown in the figure. The temperature of plate A is 300 °C. Plate B is at 100 °C, and it is centered parallel to plate A. Calculate the net radiation heat transfer from plate A to plate B.

Solution Since plate A is much bigger than plate B and since plate B is centered while facing plate A, it is reasonable to assume that all the radiation from plate B will reach plate A. Hence, the view factor from B to A is, FBA = 1.0. Since TA > TB, the net radiation heat transfer will be from A to B. Calculate the view factor from A to B by using the reciprocity rule (Eq. 4.62). Ai F ij = Aj F ji ) F AB =

AB 8 cm × 8 cm ð1:0Þ = 0:40 F = AA BA 8 cm × 20 cm

Calculate the absolute temperatures of plates A and B.

4.5

Radiation

311

T A = 300 ° C þ 273 ° = 573 K

T B = 100 ° C þ 273 ° = 373 K

Calculate the net radiation heat transfer from A to B using Eq. 4.64. qAB =

σ T A4 - T B4 1 - εA 1 1 - εB þ þ εA AA AA F AB ε B AB

Since both the plates are black, εA = 1.0 and εB = 1.0. The preceding equation can be simplified and used as shown. qAB = σAA F AB T A 4 - T B 4 = 5:669 × 10 - 8

W ð0:08 m × 0:20 m Þð0:4Þ m2  K4

× ð573 KÞ4 - ð373 KÞ4 = 32:09 W

4.5.8

Correction for Thermocouple Readings

Thermocouples, used in measuring the temperature of hot gases, are usually situated inside a thermowell. In this situation, the temperature indicated by the thermocouple, TTC, is actually an equilibrium temperature rather than the actual temperature of the gas, TG [5–7]. The hot gas transfers heat to the thermowell by convection and the thermowell in turn dissipates the heat to the surroundings (at temperature TS), such as the pipe wall, by radiation. The steady-state energy balance is Heat transferred by the gas to the thermocouple by convection = heat dissipated by the thermocouple to the surroundings by radiation hðT G- T TC Þ = σεTC T TC 4- T S 4

ð4:66Þ

Example 4.14 A thermocouple used in measuring the temperature of a hot gas in a duct indicates a temperature of 300 °C. The emissivity of the thermocouple material is 0.45 and the wall temperature of the duct is 70 °C. The heat transfer coefficient for the gas is 355 W/m2.K. Determine the actual gas temperature and the percentage error in the thermocouple measurement.

312

4

Heat Transfer

Solution Calculate the absolute temperatures of the thermocouple reading and the surrounding walls. T TC = 300 ° C þ 273 ° = 573 K

T S = 70 ° C þ 273 ° = 343 K

Substitute the known values and calculate the gas temperature, TG, using Eq. 4.66. hðT G- T TC Þ = σεTC T TC 4- T S 4 ) TG =

σεTC h

T TC 4 - T S 4 þ T TC W ð0:45Þ m2  K4 W 355 2 m K

5:669 × 10 - 8 =

ð573 KÞ4 - ð343 KÞ4 þ 300 ° C

= 306:75 ° C

Note: There is no need to convert the temperatures to absolute values for convection heat transfer (on the left hand side of the equilibrium equation, Eq. 4.66) since ΔT ° C = ΔT K Calculate the percent error in the thermocouple measurement. Percent error =

306:75 ° C - 300 ° C T G - T TC × 100 = 2:20% × 100 = 306:75 ° C TG

Practice Problems Practice Problem 4.1 2.85 kg/s of liquid lithium at an average bulk temperature of 650 °C flows in a 52.5 mm ID pipe. The properties of liquid lithium at the average bulk temperature are: density = 457 kg/m3, viscosity = 3.69 × 10-4 Pa.s, heat capacity = 4.169 kJ/kg.K, and thermal conductivity = 55 W/m.K. The heat transfer correlation applicable in this case is Nu = 5 + 0.0025Pe0.8,where Pe is the Peclet number. Peclet number is the product of Reynolds number and Prandtl number. For this situation, determine:

Practice Problems

313

A. the Peclet number. B. the heat transfer coefficient.

Practice Problem 4.2 A tank containing a process liquid is experiencing a heat loss of 2.25 kW. The temperature of the process liquid is maintained at 120 °C by using condensing steam at 300 kPa circulating in a coil immersed in the process liquid. The heat transfer coefficient of the condensing steam is 4310 W/m2.°C and the heat transfer coefficient from the coil exterior to the process liquid is 650 W/m2.°C. Fouling resistances can be ignored. If the outer diameter of the coil tubes is 35 mm, determine the length of the coil required.

Practice Problem 4.3 An overhead condenser processing the top product from a distillation column uses cooling water at 75 °F as the cooling fluid. The top product is a vapor at a temperature of 150 °F with a mass flow rate of 1150 lbm/hr., and the enthalpy of condensation is 255 Btu/lbm. The overall heat transfer coefficient used in the design of the condenser is 165 Btu/hr-ft2-°F and the temperature rise of the cooling water is 15 °F. Determine the heat transfer area required in the condenser.

Practice Problem 4.4 A 1–2 shell and tube heat exchanger with a heat exchanger surface area of 28.2 ft2 is used in cooling hot oil with a mass flow rate of oil 10,000 lbm/hr from 200 °F to 140 °F. Cooling water at 60 °F and a mass flow rate of 16,500 lbm/hr is used for cooling the oil. The overall design heat transfer coefficient is 125 Btu/hr-ft2-°F. The specific heats of water and oil are 1.0 Btu/lbm-°F and 0.55 Btu/lbm-°F respectively. The overall fouling factor used in the design of the exchanger is 0.003 hr-ft2-°F/Btu. Assuming that the mass flow rates, and the entrance temperatures of oil and water will remain the same, A. Determine the exit temperatures of oil and water when the heat exchanger is first put into service. B. Compare the heat duty under clean conditions and under fouled conditions.

314

4

Heat Transfer

Practice Problem 4.5 A steel pipe (ID = 12 in, OD = 12.75 in) carries steam at a temperature of 500 °F. The pipe is insulated with 1.50 in. thick calcium silicate insulation and the surface temperature of insulation is 120 °F. The pipe is located in a facility where the walls of the enclosure are at 80 °F. The emissivity of the insulation surface is 0.75. Calculate the heat loss per foot length of the insulated pipe due to radiation.

Solutions to Practice Problems Practice Problem 4.1 Solution A. Calculate the velocity of liquid lithium by using the continuity equation (Eq. 3.10). m_ m_ v= = = ρA ρ π D 2 i 4

2:85 457

kg m3

kg s

π 1m 52:5 mm × 4 1000 mm

= 2:88 m=s

2

Calculate the Reynolds number for liquid lithium using Eq. 3.11.

D vρ Re = i = μ

52:5 mm ×

1m m 2:88 1000 mm s -4 3:69 × 10 Pa:s

457

kg m3

= 1:872 × 105

m2 kg m2 kg m2 kg kg × 3 × 3 × 3 s s s m m m m Note on units:

 s ðdimensionlessÞ
Ns kg kg  m  s Pa  s ms m2 s2  m 2 Calculate the Prandtl number for liquid lithium using the formula given in Table 4.1.

cp μ Pr = = k

4:169 × 103

J kg  K 55

3:69 × 10 - 4 Pa  s

W mK

= 0:028

Solutions to Practice Problems

315

J kg J × kg m  s m  s Note on units: ðdimensionlessÞ
W J m ms Calculate the Peclet number by multiplying the Reynolds number and Prandtl number. Pe = Re  Pr = 1:872 × 105 ð0:028Þ = 5242 B. Calculate the heat transfer coefficient by using the given correlation for liquid metal heat transfer. Nu = 5 þ 0:0025Pe0:8 = 5 þ ð0:0025Þð5242Þ0:8 = 7:363 Calculate the heat transfer coefficient for liquid lithium by using the definition of Nusselt number (Table 4.1). The characteristic dimension, L, in case of a pipe is the inside diameter, Di. Nu = 7:363 =

hDi ) k

W 7:363 × k ð7:363Þ 55 m  K = = 7714 W=m2  K h= 0:0525 m Di

Practice Problem 4.2 Solution Subscript “pl” represents the process liquid and subscript “s” represents steam. Since fouling resistances can be ignored, calculate the overall heat transfer coefficient using Eq. 4.42. 1 1 1 þ = = U hs hpl

1

W 4310 2 m  °C ) U = 564:8 W=m2  ° C

þ

1 W 650 2 m  °C

= 0:00177 m2  ° C=W

The temperature difference driving force in this case is the difference between the saturation temperature of steam 300 kPa and the temperature of the process liquid to

316

4 Heat Transfer

be maintained at 120 °C. Determine the saturation temperature of steam at 300 kPa from steam tables or from any online steam properties resource. T s = T sat ð300 kPaÞ = 133:52 ° C Source: Spirax Sarco Steam Tables, https://www.spiraxsarco.com/resources-anddesign-tools/steam-tables/saturated-water-line From heat balance, Heat duty of the coil = heat lost by the process liquid: Rearrange the heat exchanger design equation (Eq. 4.44) to obtain a formula for the heating coil surface area required. Substitute all the known values, including 1.0 for the MTD correction factor, F, applicable in this case and calculate the heating coil surface area required. As =

q q = UFΔT UF T s - T pl 2:25 kW ×

=

1000 W kW

W ð1:0Þð133:52 ° C - 120 ° CÞ m2  ° C = 0:2946 m2 564:8

Calculate the length of the coil by using the heat exchanger surface area required, As. L=

As = πD

0:2946 m2 = 2:68 m 1m ðπ Þ 35 mm × 1000 mm

Practice Problem 4.3 Solution Subscript “v” represents the overhead vapor and subscript “w” represents cooling water. Calculate the heat duty of the condenser using the mass flow rate of the vapor and the enthalpy of condensation.

Solutions to Practice Problems

317

q = m_ v hcondn: = 1150

lbm hr

255

Btu = 293250 Btu=hr lbm

Calculate the exit temperature of cooling water. T exit,w = T inlet,w þ ΔT w = 75 ° C þ 15 ° C = 90 ° C The exit temperature of the condensate is the same as the inlet temperature of the vapor, that is, 150 °C, since condensation is an isothermal phase-change process. Calculate the log mean temperature difference by using the temperature schematic shown below and Eq. 4.36. 1 150 ° F 90 ° F ΔT1 = 60 ° F ΔT LMTD =

ΔT 2 - ΔT 1 ln

ΔT 2 ΔT 1

2 ! 150 ° F Water 75 ° F ΔT2 = 75 ° F

Vapor

=

75 ° F - 60 ° F = 67:22 ° F °F ln 75 60 ° F

Rearrange the heat exchanger design equation (Eq. 4.44) to obtain a formula for the heat transfer surface area required for the condenser. Substitute all the known values, including 1.0 for the MTD correction factor, F, applicable for condensers, and calculate the surface area required. q = As = UFΔT LMTD

293250

Btu hr

Btu 165 ð1:0Þð67:22 ° FÞ hr - ft2 - ° F

= 26:44 ft2

Practice Problem 4.4 Solution When the heat exchanger is first put into service, it operates under clean conditions with no fouling. However, the design overall coefficient, UD, includes the fouling factor. Since the overall heat transfer coefficient under clean conditions, UC, is higher than UD, there will be greater heat transfer under clean conditions resulting in different exit temperatures for both the fluids. Calculate the overall heat transfer coefficient under clean conditions using Eq. 4.43.

318

4

1 1 = - Rf = UC UD

Heat Transfer

hr - ft2 - ° F 1 - 0:003 Btu Btu 125 2 hr - ft - ° F

= 0:005 hr - ft2 - ° F=Btu UC =

1 = 200 Btu=hr - ft2 - ° F hr - ft2 - ° F 0:005 Btu

Calculate the heat capacity rates of oil and water. m_ oil cp,oil = 10000

lbm hr

m_ water cp,water = 16500

0:55

lbm hr

Btu = 5, 500 Btu=hr - ° F lbm - ° F

1:0

Btu = 16, 500 Btu=hr - ° F lbm - ° F

Therefore, C min = 5500 Btu=hr - ° F and C max = 16500 Btu=hr - ° F Calculate the NTU under clean conditions using Eq. 4.51.

U A NTU = C s = Cmin

200

Btu 28:2 ft2 hr - ft2 - ° F = 1:025 Btu 5500 hr - ° F

Calculate the heat capacity rate ratio using Eq. 4.52. Btu 5500 C min hr - ° F = 0:3333 Cr = = Btu Cmax 16500 hr - ° F Determine the heat exchanger effectiveness from Fig. 4.10b using NTU = 1.03 and Cr = 0.3333 as parameters as shown in the figure.

Solutions to Practice Problems

319

The heat exchanger effectiveness is ε = 57 % = 0.57. Water is the cold fluid and oil is the hot fluid. Calculate ΔTc and ΔTh using Eq. 4.49. Btu εCmin ðT h,in - T c,in Þ ð0:57Þ 5500 hr - ° F ð200 ° F - 60 ° FÞ = ΔT c = Btu Cc 16500 hr - ° F = 26:6 ° F Btu ε Cmin ðT h,in - T c,in Þ ð0:57Þ 5500 hr - ° F ð200 ° F - 60 ° FÞ ΔT h = = Btu Ch 5500 hr - ° F = 79:8 ° F Calculate the exit temperatures of water and oil using ΔTc and ΔTh. T e, water = T i, water þ ΔT c = 60 ° F þ 26:6 ° F = 86:6 ° F T e,oil = T i,oil - ΔT h = 200 ° F - 79:8 ° F = 120:2 ° F B. When the heat exchanger is first put into service, the actual heat transfer is Btu ð26:6 ° FÞ hr - ° F = 438,900 Btu=hr

qactual = C c ΔT c =

16500

320

4

Heat Transfer

Under fouled conditions, the exit oil temperature is 140 °F, therefore the heat duty can be calculated using the heat balance equation (Eq. 4.35). lbm Btu 0:55 ð200 ° F - 140 ° FÞ hr lbm - ° F = 330, 000 Btu=hr

qfouled = m_ oil cp,oil ΔT oil =

10000

As expected, the heat duty under clean conditions is much higher than the heat duty under fouled conditions.

Practice Problem 4.5 Solution The radiation occurs from the outside surface of insulation covering the pipe. The OD of the insulated surface is Do = 12.75 in. + 2(1.50 in.) = 15.75 in. Calculate the outside surface area of the pipe in terms of L, the length of the pipe. Ao = πDo L = π 12:75 in ×

1 ft L = 4:123L ft2 12 in

Determine the absolute temperatures of the two bodies. Body 1 is the insulated pipe, and the enclosure is body 2. ∘

T 1 = 120 F þ 460∘ = 580∘ R

T 2 = 80∘ F þ 460∘ = 540∘ R

All the radiation from the pipe will reach the walls of the enclosure. Therefore, the view factor is, F12 = 1.0. The surface area of the enclosure will be much greater than the surface area of the pipe, that is, A2 >>> A1 and the term 1ε-2 Aε22 can be neglected relative to the other terms in the denominator of Eq. 4.64. Substitute the preceding results and the known values into Eq. 4.64.

References

321

σðT 1 4 - T 2 4 Þ = σA1 ε1 ðT 1 4 - T 2 4 Þ 1 - ε1 1 þ ε1 A 1 A1 Btu = 0:1714 × 10 - 8 ð4:123L ft2 Þð0:75Þ hr - ft2 - ° R4

q12 =

× ð580 ° RÞ4 - ð540 ° RÞ4 = 149:12L Btu=hr The heat loss per unit length is,

q12 = 149:12 Btu=hr - ft L

References 1. T.L. Bergman, A.S. Lavine, F.P. Incropera, P. DeWitt David, Fundamentals of Heat and Mass Transfer, 8th edn. (Wiley, USA, 2019) 2. C.S. Bildea, A.C. Dimian, PVC Manufacturing Process by Suspension Polymerization (Appendix B: Heat Exchanger Design, Wiley Online Library, 2008) Download from https:// onlinelibrary.wiley.com/doi/pdf/10.1002/9783527621583.app2 3. F. Coletti, Editor, Heat Exchanger Design Handbook Multimedia Edition (Begell Digital Portal) 4. C. Ezgi, Basic Design Methods of Heat Exchangers (Intech Open, 2017) Download from, https:// www.intechopen.com/chapters/54521 5. A.M. Flynn, T. Akashige, L. Theodor, Kern’s Process Heat Transfer, 2nd edn. (Wiley, USA, 2019) 6. N.S. Nandagopal, Fluid and Thermal Sciences – A Practical Approach for Students and Professionals (Springer, Cham, 2022) 7. C.R. Shaddix, A new method to compute the proper radiant heat transfer correction of bare wire thermocouple measurements, in Proc. of 10th U.S. National Combustion Meeting, (2017) 8. S. Whitaker, Forced convection heat transfer correlations for flow in pipes, past flat plates, single cylinders, single spheres, and for flow in packed beds and tube bundles. AICHE J. 18(2), 361–371 (1972)

Chapter 5

Mass Transfer

5.1

Introduction

Mass transfer is a term used to describe the movement of a substance or component in a mixture from a region of high concentration to one of lower concentration [5]. This movement of species can occur within a given phase or between different phases, which is more common in chemical engineering applications [6]. This is called interphase mass transfer. A good example of interphase mass transfer is gas absorption, where a non-desirable component is absorbed by a liquid solvent involving the diffusion of gas molecules from the bulk gas phase into the bulk liquid phase [6]. Mass transfer is the basis for separation processes, which play a crucial role in separation of desirable products from a product mixture from a synthesizing reaction or from natural resources. Atmospheric distillation of crude oil is a widely used, excellent example of separation process applied to a natural resource [1]. Another example of separation process is separation of ethylene after dehydrogenation of ethane produces ethylene, unreacted ethane, and undesired species such as methane. The process of drying, whether naturally occurring or induced in a hightemperature dryer, involves the transfer of water molecules from a solid substrate into a gas phase, usually, atmospheric air. The process of drying is actually an example of simultaneous heat and mass transfer (SHMT). Heat is transferred to the moist substrate enabling the vaporization of water molecules, which is a diffusionbased mass transfer process [2]. Mass transfer operations are sometimes labeled as unit operations. Besides the mass transfer operations already mentioned, some of the other unit operations [3] are filtration, crystallization, liquid extraction, and leaching.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 N. S. Nandagopal, Chemical Engineering Principles and Applications, https://doi.org/10.1007/978-3-031-27879-2_5

323

324

5.2

5

Mass Transfer

Molecular Diffusion

Molecular diffusion is caused by the spontaneous, random motion of molecules from a region of higher concentration to a region of lower concentration. The rate of diffusion is governed by Fick’s law of diffusion, which states that the molar flux of the diffusing component (moles per unit area per unit time) is proportional to the concentration gradient in the direction of diffusion [1, 6]. The proportionality relationship can be transformed into an equation by introducing a proportionality constant known as mass diffusivity or diffusion coefficient. The diffusion coefficient is usually represented by DAB, which should be interpreted as the diffusivity of component A through a layer of non-diffusing (or stagnant) component B. Fick’s law can be mathematically represented by the following equation. N A = ð- DAB Þ

dCA dz

ð5:1Þ

The following nomenclature is used in Eq. 5.1. NA = molar flux of A (mol/cm2.s, kmol/m2.s, lbmol/ft2.sec) DAB = diffusion coefficient of component A into stagnant B (cm2/sec, m2/sec, ft2/sec) dCA dz = concentration gradient in the z direction, the direction of diffusion, where, CA is in mol/cm3, mol/L, kmol/m3, or lbmol/ft3, and z is in cm, m, or ft The negative sign in Eq. 5.1 is necessary because the concentration gradient is always negative in the direction of diffusion and the molar flux must be positive (similar to negative sign in Fourier’s law of heat conduction). The diffusion coefficient, DAB, depends on the nature of collisions between species A and B. The diffusivity value is normally inversely proportional to the pressure, and directly proportional to T 3/2. A common example of diffusion of A into stagnant B is the evaporation of hot water into surrounding air. Here, water vapor is the diffusing component and air is the stagnant, non-diffusing component. Example 5.1 The diffusion of sulfur dioxide in air is being studied as part of pollution control strategies. At the source, the mass concentration of sulfur dioxide was measured to be 44 mg/m3 and 2 m away from the source, the concentration was 21 mg/m3. Experimental studies indicate a diffusivity of 0.1321 cm2/sec for sulfur dioxide in air. Determine the molar flux of sulfur dioxide in air. Solution The molecular weight of sulfur oxide is M SO2 = 64 g=mol. Using the molecular weight of sulfur oxide, calculate the molar concentrations of sulfur dioxide at the source and at the point of interest as shown.

5.3

Mass Transfer Coefficients and Interphase Mass Transfer

C A1 = C A2 =

44 mg m3 ×

1g 1000 mg g 64 mol

×

1 m3 106 cm3

21 mg m3 ×

1g 1000 mg g 64 mol

×

1 m3 106 cm3

325

= 6:875 × 10 - 10 mol=cm3 = 3:281 × 10 - 10 mol=cm3

Substitute all the known values into the simple integrated form of Fick’s law and calculate the molar flux as shown. dC A ΔCA ) N A = ð - DAB Þ dz Δz ðC A2 - C A1 Þ ) N A = ð - DAB Þ ) ðz2 - z1 Þ mol mol 3:281 × 10 - 10 3 - 6:875 × 10 - 10 3 2 cm cm cm N A = - 0:1321 100 cm s 2 m× -0 m = 2:374 × 10 - 13 mol=cm2  s N A = ð - DAB Þ

5.3

Mass Transfer Coefficients and Interphase Mass Transfer

The driving force for mass transfer is concentration difference in liquids and difference in partial pressure in the gas phase. This is analogous to the temperature difference driving force in heat transfer and the pressure difference driving force in momentum transfer (fluid mechanics). Any transport process can be modeled in terms of the transfer coefficient and the driving force. Accordingly, the mass or molar flux of a diffusing component is the product of the mass transfer coefficient times the driving force, that is concentration or partial pressure difference. The mass transfer coefficient is usually represented by lower case letter, k. Appropriate subscripts are added depending on the nature of the driving force. For example, kc represents the mass transfer coefficient when the driving force is concentration difference. The units of the mass transfer coefficient also depend on the units of the driving force as shown here [6]. The molar flux can be written in terms of the mass transfer coefficient and the driving force as shown in the following equation.

326

5

Mass Transfer

Table 5.1 Units for mass transfer coefficients Flux equation NA = kcΔCA NA = kpΔPA NA = kxΔxA NA = kyΔyA

Driving force Concentration difference (liquid phase)

Symbol ΔCi

Mass transfer coefficient kc

Partial pressure difference (gas phase) Mole fraction difference (liquid phase) Mole fraction difference (gas phase)

ΔPi

kp

Δxi

kx

kmol=m2 :s kmol=m3 soln kmol=m2 :s kPa kmol/m2. s

Δyi

ky

kmol/m2. s

N A = kc ΔCA

SI units of MTC

ð5:2Þ

Rearrange Eq. 5.2 and substitute the appropriate units to obtain the units for the mass transfer coefficient. kc =

mol=cm2 :s kmol=m2 :s lbmol=ft2 : sec NA ,  , ΔC A mol=L soln kmol=m3 soln lbmol=ft3 soln

The appropriate units for the mass transfer coefficients for the different types of driving forces are summarized in Table 5.1. The mass transfer coefficients represented in Table 5.1 are individual mass transfer coefficients in the sense that they represent the coefficient in a particular phase [6]. However, most chemical engineering applications involve interphase mass transfer requiring the use of overall mass transfer coefficient, described in detail later in this section.

5.3.1

Interphase Mass Transfer

In interphase mass transfer, the transport of species occurs across two or more phases. The most commonly occurring type of interphase mass transfer is between liquid and gas phases. In this case, there are two individual resistances to mass transfer, one in the liquid phase and another in the gas phase [1, 6]. Consider the example of absorption of a gas species from a gas mixture by a liquid solvent. There is an interface between the gas and liquid phases. The driving force for the diffusing species in the gas phase is the difference between the partial pressure of the species in the bulk gas phase and the partial pressure at the interface. The driving force for the diffusing species in the liquid phase is the difference between the concentration of the species at the interface and the concentration of the species in the bulk liquid phase. The preceding concept is illustrated in Fig. 5.1. The figure and the nomenclature in it are self-explanatory. At steady state, the molar flux of the diffusing component will be equal in both the gas and liquid phases and

5.3

Mass Transfer Coefficients and Interphase Mass Transfer

327

Fig. 5.1 Driving forces for interphase mass transfer

the molar flux can be written by multiplying the driving force by the appropriate individual mass transfer coefficient as shown in Eq. 5.3. In the following equation, the typical units of the variables are: PAG, PAi (atm, kPa, psi) and CAi, CAL (mol/cm3, mol/L, kmol/m3, lbmol/ft3) N A = kG ðPAG- PAi Þ = k L ðC Ai- C AL Þ

ð5:3Þ

In Eq. 5.3, NA: molar flux of A (kmol/m2.s, lbmol/ft2-sec) kG: individual gas phase mass transfer coefficient [kmol/m2.s.kPa, lbmol/ft2-sec- psi] PAG,PAi: partial pressures of A in the bulk gas phase and at the interface respectively kL: individual liquid phase mass transfer coefficient [kmol/m2.s.(kmol/m3), lbmol/ ft2-sec-(lbmol/ft3)] CAL,CAi: concentrations of A in the bulk liquid phase and at the interface, respectively It is difficult to measure/estimate the partial pressure and concentration of the diffusing species at the interface. This issue is addressed by using overall liquid and gas phase mass transfer coefficients and by using equilibrium partial pressure and concentration, which means that the gas–liquid equilibrium relationship must be considered [1, 6]. The overall coefficients are represented by the corresponding upper case letters and the equilibrium quantities are represented by using the ‘*’ superscript. Recall from heat transfer that the inverse of the convection heat transfer coefficient is the resistance to convection heat transfer rate per unit area. The molar flux in mass transfer is similar to heat transfer per unit area (heat flux) in heat transfer. Also, in heat transfer, the overall convection heat transfer resistance (which is the reciprocal of the overall heat transfer coefficient, U ) is the sum of the individual inside and outside heat transfer resistances, that is, U1 = h1i þ h10 . The same approach can also be used for interphase mass transfer, that is, the overall mass transfer resistance is the sum of the individual mass transfer resistances. However, in contrast to heat transfer, mass transfer involves two phases which necessitates the use of gas–liquid equilibrium relationship constant.

328

5

Mass Transfer

The following equations can be used to calculate the overall liquid and gas mass transfer coefficients. 1 m 1 = þ K G kG kL

ð5:4Þ

1 1 1 = þ K L k L mk G

ð5:5Þ

In Eqs. 5.4 and 5.5, KG: overall gas mass transfer coefficient [kmol/m2.s.(kPa), lbmol/ft2-sec-(psi)] KL: overall liquid mass transfer coefficient [kmol/m2.s.(kmol/m3), lbmol/ft2-sec(lbmol/ft3)] kG, kL: individual gas and liquid phase mass transfer coefficients as described in the nomenclature for Eq. 5.3. m: gas–liquid equilibrium relationship constant, the equilibrium relationship being, PA = mC A

ð5:6Þ

In Eq. 5.4, if m is small, then, KG  kG, and the mass transfer is gas phase controlled. m will be small when the gas is highly soluble in the liquid solvent. In Eq. 5.5, if m is large, then, KL  kL, and the mass transfer is liquid phase controlled. m will be large when the gas has low solubility in the liquid solvent. The molar flux can be calculated by using the overall mass transfer coefficient and Eq. 5.7 [6]. N A = K G PAG- PA = K L C A - C AL

ð5:7Þ

In Eq. 5.7, PA is the partial pressure of A in equilibrium with the concentration of A in the bulk liquid phase, that is, PA = mC AL

ð5:8Þ

and C A is the concentration of A in the bulk liquid phase that is in equilibrium with the partial pressure of A in the bulk gas phase, that is, C A =

PAG m

ð5:9Þ

PA and C A are illustrated in Fig. 5.2. Example 5.2 In a wetted-wall stripping tower, ammonia is stripped from an ammonia–water solution using a counterflowing air stream. The overall liquid mass transfer coefficient is KL = 0.00025 kmol/ min . m2(kmol NH3/m3soln). At a certain location in the

5.3

Mass Transfer Coefficients and Interphase Mass Transfer

329

Fig. 5.2 Equilibrium concentration and partial pressure

tower, the bulk concentration in the dropping liquid was measured to be 0.0058 kmol NH3/m3soln and the partial pressure of ammonia in the gas stream was 0.03 atm. At the operating conditions, the equilibrium relationship is PAi = 2.867CAi, PAi(atm), CAi(kmol NH3/m3soln). If 75% of the total resistance to mass transfer is in the gas phase, determine the following: A. kL and kG B. KG C. PAi and CAi Solution Determine m from the given equilibrium relationship, PAi = 2.867CAi, ) m = 2:867 atm= kmol NH3 =m3 soln A. Since 75% of the overall resistance to mass transfer is in the gas phase, the following equation, based on Eq. 5.5, K1L = k1L þ mk1 G , can be used in calculating kG and kL. 1 1 = ð0:75Þ KL mk G

1 1 = ðmÞð0:75Þ ) KL kG kmol 0:00025 min  m2 ðkmol NH3 =m3 solnÞ KL kG = = ð0:75ÞðmÞ atm ð0:75Þ 2:867 kmol NH3 =m3 soln )

= 0:00012 kmol= min  m2  ðatmÞ Since 75% of the overall resistance to mass transfer is in the gas phase, 25% of the overall resistance to mass transfer is in the liquid phase. Therefore,

330

5

1 1 = ð0:25Þ kL KL kL =

Mass Transfer

)

KL kmol = 4K L = 4 0:00025 0:25 min  m2 ðkmol NH3 =m3 solnÞ

= 0:001 kmol= min  m2 kmol NH3 =m3 soln B. Calculate KG using Eq. 5.4. 1 KG

=

1 m þ kG kL

atm kmol NH3 =m3 soln = þ kmol kmol 0:00012 0:001 min  m2  ðatmÞ min  m2 ðkmol NH3 =m3 solnÞ = 11200 min  m2  ðatmÞ=kmol 2:867

1

) K G = 0:00009 kmol= min  m2  ðatmÞ C. To calculate, PAi and CAi, first calculate the molar flux using Eq. 5.7. Prior to using Eq. 5.7, calculate PA using Eq. 5.8.

PA = mC AL =

2:867

= 0:0166 atm

atm kmol NH3 m3 soln

0:0058

kmol NH3 m3 soln

N A = K G PAG - PA = 0:00009

kmol ð0:03 atm - 0:0166 atmÞ min  m2  ðatmÞ

= 1:21 × 10 - 6 kmol= min  m2 Determine PAi and CAi using Eq. 5.3.

5.4

Gas Absorption and Stripping

331

N A = kG ðPAG - PAi Þ = k L ðC Ai - CAL Þ ) kmol 1:21 × 10 - 6 NA min  m2 = 0:0199 atm PAi = PAG = 0:03 atm kmol kG 2 0:00012 min  m atm kmol 1:21 × 10 - 6 NA min  m2 þ 0:0058 kmol NH3 þ C AL = C Ai = kmol kL m3 soln 2 0:001 min  m kmol NH3 m3 soln 3 = 0:0070 kmol NH3 =m soln

5.4

Gas Absorption and Stripping

Gas absorption is extensively used in the chemical industry for the purification of gas streams by absorbing non-desirable component(s) from the gas stream using a suitable solvent. A popular example is the absorption of ammonia gas from an air stream using water as the solvent. Absorption and stripping are often used in tandem, where the stripping operation is used in removing the absorbed gas from the solvent to facilitate the regeneration and reuse of the solvent. A classic example is the removal of sour gas (hydrogen sulfide) from natural gas using amine as the solvent. The rich amine with the absorbed sour gas is sent to a stripping tower, where the sour gas is removed from the rich amine solution using steam as the stripping agent. After the stripping operation, the lean amine can be used for the absorption of sour gas in the absorption tower. Note: Calculations related to stripping operation are illustrated in Example 5.5. The design of absorption/stripping columns involves two aspects, process design and equipment design. Process design involves calculation of the following parameters [1, 6]: • The minimum liquid to gas flow ratio so that the absorption tower can operate at 1.3–1.5 the minimum L/G ratio in practice. • The mole fraction of the absorbed (solute) gas in the gas leaving the tower. • The concentration of the solute gas in the liquid solvent leaving the tower. • The height of a transfer unit (HTU). • The number of transfer units required to accomplish the desired absorption (NTU). • The height of packing required in a packed absorption column. Equipment design involves determining the appropriate diameter of the column using generalized pressure drop correlations (GPDC), which was considered in the fluid mechanics [3] chapter (Chap. 3, Sect. 3.11).

332

5.4.1

5

Mass Transfer

Process Design of Absorption Columns

Absorption columns in the process industry commonly use a counterflow operation, where the solvent is introduced from the top of the column and the gas flows from the bottom of the column, opposite to the flow of the liquid. A schematic diagram of a typical absorption tower is shown in Fig. 5.3. The molar flux is the molar flow rate per cross-sectional area of the tower. The following nomenclature is used in Fig. 5.3. Section 1-1: Bottom of the tower, where gas enters, and liquid leaves the tower. Section 2-2: Top of the tower, where the gas leaves, and the liquid enters the tower. G1: molar flux of the gas entering with more solute (lbmol/hr-ft2) G2: molar flux of the gas leaving with less solute (lbmol/hr-ft2) L2: molar flux of the liquid solvent entering with less or no solute (lbmol/hr-ft2) L2: molar flux of the liquid solvent leaving with more solute (lbmol/hr-ft2) LS: molar flux of the solute-free liquid entering and leaving (lbmol/hr-ft2) GS: molar flux of the solute-free gas entering and leaving (lbmol/hr-ft2) xA1: mole fraction of the solute in the liquid leaving the tower (moles solute/moles liquid solution) xA2: mole fraction of the solute in the liquid entering the tower (moles solute/moles liquid solution) xA2 = 0, for pure solvent yA1: mole fraction of the solute in the gas entering the tower (moles solute/moles gas mixture) yA2: mole fraction of the solute in the gas leaving the tower (moles solute/moles gas mixture) XA1: mole ratio in the liquid leaving the tower (moles solute/moles solute-free liquid) XA2: mole ratio in the liquid entering the tower, XA2 = 0 for pure solvent (moles solute/moles solute-free liquid) Fig. 5.3 Counterflow absorption tower

5.4

Gas Absorption and Stripping

333

YA1: mole ratio in the gas entering the tower (moles solute/moles solute-free gas) YA2: mole ratio in the gas leaving the tower (moles solute/moles solute-free gas) The relationship between the mole fractions and mole ratios are xA 1 - xA yA YA = 1 - yA

XA =

ð5:10Þ ð5:11Þ

The material balance equations for the diffusing component, A, are all based on the following mole balance equation [6]. moles of A entering the tower = moles of A leaving the tower

ð5:12Þ

G1 yA1 þ L2 xA2 = G2 yA2 þ L1 xA1

ð5:13Þ

GS Y A1 þ LS X A2 = GS Y A2 þ LS X A1 )

ð5:14Þ

GS ðY A1- Y A2 Þ = LS ðX A1- X A2 Þ )

ð5:15Þ

ðY - Y A2 Þ LS = A1 GS ðX A1 - X A2 Þ

ð5:16Þ

Equation 5.16 is a very important equation and it is used in calculating the actual liquid to gas molar flow ratio. In most practical situations, the entering gas flow rate and composition are known, the amount of solute to be removed from the gas is specified, and pure solvent is used. This means that Gs, YA1, YA2, and XA2(=0) are known quantities or they can be calculated from the given information. Therefore, if the actual liquid to gas flow ratio is known, then the composition of the liquid leaving the tower can be determined. The actual liquid to gas flow ratio is usually 1.3–1.5 times the minimum liquid to gas flow ratio, (LS/GS)min, which can be determined by using the equilibrium relationship as shown here. Equation 5.16 can be represented by a straight line relationship, Y = GLSS X, where GLSS is the slope of the line, known as the operating line. The slope of the actual operating line can be calculated by using Eq. 5.16. The driving force in mass transfer, as well as in gas absorption, is the difference between the actual gas composition and the equilibrium gas composition, that is, Y - Y, and when the operating line touches the equilibrium curve, Y = Y, and the driving force is zero implying an infinitely tall tower required to accomplish the required absorption of the solute from the gas stream. This condition represents the absolute minimum liquid flow rate required and hence a liquid flow rate higher than the minimum required must be used to accomplish the desired results. The preceding concepts can be used in determining the minimum liquid to gas flow ratio both analytically and graphically as described below. (LS/GS)min can be determined by substituting the liquid composition that is in equilibrium with the

334

5

Mass Transfer

Fig. 5.4 Minimum and actual liquid to gas flow ratio

entering gas composition in Eq. 5.16. If the equilibrium relationship is, Y = mX, then, X A1 =

Y A1 m

ð5:17Þ

Hence, LS GS

= min

ðY A1 - Y A2 Þ X A1 - X A2

ð5:18Þ

(LS/GS)actual and (LS/GS)min in conjunction with the equilibrium line are illustrated in Fig. 5.4. Example 5.3 A trayed tower with a diameter of 2.75 ft uses pure water to absorb ammonia from an air stream with a mass flow rate of 15,000 lbm/hr. The mole fraction of ammonia in the air stream is 0.04 and the desired mole fraction of ammonia in the exit air stream is 0.001. The equilibrium relationship is Y = 1.91X where Y is the mole ratio of ammonia to air in the gas stream and X is the mole ratio of ammonia to water in the liquid stream. Determine: A. the mass flow rate of water required if the tower is designed to operate at 1.4 times the minimum liquid flow rate. B. the mole fraction of ammonia in water leaving the tower. Solution Draw the schematic diagram for the process with nomenclature consistent with that used in Fig. 5.3.

5.4

Gas Absorption and Stripping

335

A. Calculate the molar flux of the air–ammonia gas mixture entering the absorption tower using the molecular weight of air as an approximation (since the mole fraction of air in the mixture is 1 - 0.04 = 0.96, which is almost 1). m_ G1 = G1 = M × Acs

lbm hr = 87:08 lbmol=hr‐ft2 ) π 2 ð2:75 ftÞ 4

15000 29

lbm lbmol

lbmol mix lbmol air ð1 - 0:04Þ lbmol mix hr‐ft2 = 83:60 lbmol air=hr‐ft2

GS = G1 ð1 - ya1 Þ = 87:08

Calculate YA1 and YA2 using Eq. 5.11. yA1 0:04 = = 0:0417 1 - yA1 1 - 0:04 yA2 0:001 = Y A2 = = 0:001 1 - yA2 1 - 0:001

Y A1 =

Determine the exit mole ratio of ammonia solute to water in equilibrium X A1 with the entrance mole ratio of ammonia to air (YA1) using the given equilibrium relationship.

336

5

Y A1 = 1:91X A1 ) X A1 =

Mass Transfer

0:0417 Y A1 = = 0:0218 1:91 1:91

Calculate the minimum (LS/GS)min using Eq. 5.18. LS GS

= min

ðY A1 - Y A2 Þ 0:0417 - 0:001 = = 1:87  0:0218 - 0 X A1 - X A2

With the tower operating at 1.4 times the minimum liquid flow the actual (LS/ GS)actual is LS GS

= 1:4 actual

LS GS

= 1:4 × 1:87 = 2:62 min

Calculate the actual molar flux of water feed using the (LS/GS)actual ratio. lbmol air hr‐ft2 = 219:03 lbmol water=hr‐ft2

LS, actual = 2:62GS,actual = 2:62 83:60

Calculate the mass flow rate of water required. lbmol lbm 18 lbmol hr‐ft2 = 23417 lbm=hr water

m_ L2 = L2 × M w × Acs = 219:03

π ð2:75 ftÞ2 4

B. Calculate the mole fraction of ammonia in water leaving the tower using Eq. 5.10 after first calculating the mole ratio moles ammonia to moles water using the material balance equation (Eq. 5.16).

X A1 =

xA xA1 ) 0:0155 = ) xA1 = 0:0153 1 - xA 1 - xA1

5.4

Gas Absorption and Stripping

5.4.1.1

337

Height of Packing Required, Height of a Transfer Unit (HTU), and Number of Transfer Units (NTU) for Gas Absorption Columns

The height of packing, Z, required for a specified gas absorption unit operation can be determined in terms of the height of a transfer unit (HTU) and the number of transfer units (NTU) required. HTU and NTU can be determined based on either the liquid phase or the gas phase (Eq. 5.18) and the corresponding mass transfer coefficients are used for calculations. ð5:18Þ

Z = ðNTU OL ÞðHTU OL Þ = ðNTU OG ÞðHTU OG Þ In Eq. 5.18,

Z: height of packing required (ft or m) NTUOL: number of overall transfer units required based on the liquid phase HTUOL: height of an overall transfer unit based on the liquid phase NTUOG: number of overall transfer units required based on the gas phase HTUOG: height of an overall transfer unit based on the gas phase Note: The term “overall” implies that the calculations include both the liquid phase and gas phase mass transfer resistances. The number of transfer units required depends on the difference between the entrance and exit mole fractions/concentrations of the species being absorbed/ stripped. It also depends on the driving force available. Therefore, the number of transfer units required is directly proportional to the degree of purification or separation required and inversely proportional to the driving force available. The number of transfer units required can be calculated using Eq. 5.20 or Eq. 5.21 depending on the information available [1, 6]. NTU OL =

xA1 - xA2 , xA - xA LM

xA - xA

= LM

NTU OG =

yA1 - yA2 , yA - yA LM

yA - yA

= LM

xA - xA

1

- xA - xA

2

ð5:20Þ

2

ð5:21Þ

ð x - xA Þ ln xA - x 1 ð A A Þ2 yA - yA

1

- yA - yA

ð yA - y Þ ln y - yA 1 ð A A Þ2

The denominators of the NTU formulas represent the log mean driving force (based on mole fraction differences) available for mass transfer, similar to the log mean temperature difference (LMTD) driving force in heat transfer. The height of a transfer unit required is directly proportional to the amount of gas/liquid being handled and inversely proportional to the mass transfer coefficient. The height of a transfer unit can be calculated using either Eq. 5.22 or Eq. 5.23 depending on the information available [1, 6].

338

5

HTU OL =

HTU OG =

L1 , K x að1 - xA ÞLM

ð1 - xA ÞLM =

G1 , K y að1 - yA ÞLM

ð1 - yA ÞLM =

ð1 - xA1 Þ - 1 - xA1 ln

ð1 - xA1 Þ ð1 - xA1 Þ

ð1 - yA1 Þ - 1 - yA1 ln

ð1 - yA1 Þ ð1 - yA1 Þ

Mass Transfer

ð5:22Þ

ð5:23Þ

In Eq. 5.22, Kx is the overall liquid phase mass transfer coefficient based on mole fraction driving force and it has units of lbmol/hr ‐ ft2 ‐ (mole fraction). Similarly, Ky is the overall gas phase mass transfer coefficient based on mole fraction driving force. In both Eqs. 5.22 and 5.23, ‘a’ represents the packing surface area available per unit volume of packing with units of ft2/ft3. It is the same as the packing factor, FP, defined for packings used in packed beds (as described in Sect. 3.11.2.1 in the chapter on Fluid Mechanics). It is a common practice to specify the mass transfer coefficient in terms of a capacity-based mass transfer coefficient, which is the product Kxa(as well as Kya) with units as shown here [1, 6]. Kxa  Kya 

lbmol hr‐ft ‐ðmole fractionÞ 2

ft2 ft3



lbmol hr‐ft3 ‐ðmole fractionÞ

The nomenclature used in Eqs. 5.20 through 5.23 is identical to the nomenclature used in Fig. 5.3. Example 5.4 A volatile organic vapor is to be absorbed from an air stream at 70 °F and 14.7 psia, using water as the solvent. The inlet air stream with a flow rate of 4870 cfm contains 4.25 mole% of organic vapor. The absorption column, with a diameter of 4.50 ft, has been designed for a minimum water flow rate of 1550 lbmol/hr. The overall capacity-based gas phase mass transfer coefficient is K y a = 9:85 lbmol=hr‐ft3 ‐ðmole fractionÞ: At the operating conditions, the equilibrium relationship is Y = 3.45X where Y is the mole ratio of the organic vapor to air in the gas stream and X is the mole ratio of the organic vapor to water in the liquid stream. Determine: A. the % removal of the organic vapor from air stream at minimum liquid flow rate. B. the height of an overall gas phase transfer unit. Solution Draw the schematic diagram for the process as shown. VOC represents the vapor of the volatile organic compound.

5.4

Gas Absorption and Stripping

339

A. First, calculate the molar flow rate of air using ideal gas law. Convert the air temperature to absolute value. T = 460 ° + 70 ° F = 530 ° R. 3

ft 60 min PV_ 1 ð14:7 psiaÞ 4870 min × hr N_ 1 = = 3 RT 10:73 psia‐ft0 5300 R

= 755:3 lbmol=hr

lbmol‐ R

Calculate the cross-section area of the tower. ACS =

π πD2 ð4:50 ftÞ2 = 15:90 ft2 = 4 4

Calculate the molar flux of solute-free inlet air stream. lbmol 755:3 ð1 - 0:0425Þ hr N_ 1 ð1 - yA1 Þ GS = G1 ð1 - yA1 Þ = = ACS 15:90 ft2 = 45:48 lbmol=hr‐ft2 LS, min =

1550 lbmol N_ S, min hr = = 97:48 lbmol=hr‐ft2 ACS 15:90 ft2

Calculate (LS/GS)min by substituting the known values.

340

5

LS GS

= min

Mass Transfer

97:48 lbmol=hr‐ft2 = 2:1434 45:48 lbmol=hr‐ft2

Calculate YA1 using Eq. 5.11. Y A1 =

yA1 0:0425 = = 0:0444 1 - yA1 1 - 0:0425

Using the equilibrium relationship, calculate the mole ratio of VOC to water in the exit solution that is in equilibrium with the mole ratio of VOC to carrier gas in the gas entering the tower. Y = 3:45X ) 0:0444 Y = 0:0129 X A1 = A1 = 3:45 3:45 Calculate the mole ratio of the solute (VOC) to the carrier gas, that is, YA2, in the exit air stream using Eq. 5.18. LS GS

= 2:1434 = min

Y A1 - Y A2 0:0444 - Y A2 = ) Y A2 = 0:0167 X A1 - X A2 0:0129 - 0

Calculate yA2 using Eq. 5.11. Y A2 =

yA2 yA2 ) 0:0167 = ) yA2 = 0:0164 1 - yA2 1 - yA2

Calculate the percent recovery of the solute (VOC) as shown. %recovery =

ya1 - ya2 0:0425 - 0:0164 100 = 61:41% 100 = 0:0425 ya1

B. Calculate the molar flux of the gas entering the tower (G1).

G1 =

755:3 lbmol N_ 1 hr = = 47:50 lbmol=hr‐ft2 ACS 15:90 ft2

Determine xA1, the mole fraction of the solute in the liquid leaving the tower using the material balance equation for the solute (Eq. 5.16).

5.4

Gas Absorption and Stripping

341

Since dilute solutions are being considered, the same equilibrium relationship can be used in terms of both mole ratios as well as mole fractions. Using the equilibrium relationship, calculate the mole fraction of VOC in the gas phase that is in equilibrium with the mole fraction of VOC in the solution leaving the tower. yA1 = 3:45xA1 = ð3:45Þð0:0127Þ = 0:0438 Calculate ð1 - yA ÞLM using Eq. 5.23. ð1 - yA ÞLM =

ð1 - yA1 Þ - 1 - yA1 ln

ð1 - yA1 Þ 1 - yA1

=

ð1 - 0:0425Þ - ð1 - 0:0438Þ ð1 - 0:0425Þ ln ð1 - 0:0438Þ

= 0:9568 Calculate the height of transfer unit required using the first part of Eq. 5.23. G1 HTU OG = = K y a ð1 - yA ÞLM = 5:04 ft

47:50 9:85

lbmol hr‐ft2

lbmol ð0:9568 mole frn:Þ hr‐ft ‐mole frn: 3

Note: The following example is an illustration of a stripping process. The same equations developed for gas absorption can also be used for gas stripping with proper attention and care to the signs for mole fractions and mole ratios. It is important to note that gas stripping results in the increase in mole fraction of the solute in the gas used as a stripping agent and decrease in mole fraction of the solute in the solvent, so that the solvent can be used again for gas absorption. Example 5.5 Pure air is used in stripping an organic pollutant from water. The equipment used is a counterflow packed tower with a diameter of 1.2 m, where the contaminated water is sprayed from the top of the tower and air is fed from the bottom of the tower. The mass flow rates of polluted water and pure air are 200 kg/min and 15 kg/min

342

5 Mass Transfer

respectively. Ninety percent of the pollutant, which has a mole fraction of 0.00041 in water is to be removed. The equilibrium relationship is y = 69x, where y is the mole fraction of the pollutant in the gas phase and x is the mole fraction of the pollutant in water. A. If the height of the packing is 5.3 m, determine the height of an overall liquid phase transfer unit. B. Determine the required value of the overall liquid-based capacity mass transfer coefficient, Kxa. Solution Draw the schematic diagram for the process, indicating all known information.

A. Since the mole fraction of pollutant in water is very small, calculate the molar flow rate of water entering the tower by using the molecular weight of water as a good approximation. kg 216 min m_ = 12 kmol= min N_ w2 = w2 = kg Mw 18 kmol

Calculate the cross-section area of the tower. ACS =

π ð1:20 mÞ2 = 1:131 m2 4

5.4

Gas Absorption and Stripping

343

Calculate the molar flux of solute-free water entering the tower. L2 =

12 kmol N_ w2 min = = 10:61 kmol= min  m2 ACS 1:131 m2

Since the mole fraction of the pollutant is very small and 90% of this is removed, the mole fraction of the pollutant leaving the tower with water is xA1 = ð0:1Þð0:00041Þ = 0:000041: Calculate G1 using the mass flow rate of pure air entering the tower, the molecular weight of air, and the cross section area of the tower. G1 =

kg 15 min m_ a1 = = 0:4573 kmol= min  m2 kg M a ACS 29 kmol ð1:131 m2 Þ

Determine yA2, the mole fraction of the solute in air leaving the tower using the material balance equation for the solute (Eq. 5.13). Since the composition mole fractions are very small, G2 = G1 = G and L2 = L1 = L, can be assumed without any loss of accuracy.

Determine the compositions of the water solvent in equilibrium with the entrance and exit gas streams respectively, by using the given equilibrium relationship. yA1 0 = = 0 and 69 69 y 0:00863 xA2 = A2 = = 0:000125 69 69

y = 69x ) xA1 =

Modify the Eq. 5.20 for the number of overall liquid based transfer units required, which applies for gas absorption (rich solvent leaving the tower after absorbing the solute gas), for the stripping process (lean solvent leaving the tower after solute gas has been stripped from the rich solvent). Note that subscript ‘1’ represents the bottom of the tower where the lean liquid leaves and subscript ‘2’ represents the top of the tower where the rich liquid enters. Therefore, for stripping, xA2 > xA1 and xA > xA , while the opposite is true for gas absorption.

344

5

NTU OL =

xA2 - xA1 , xA - xA LM

xA - xA

= LM

xA - xA

2

- xA - xA

Mass Transfer

1

ð xA - x Þ ln x - xA 2 ð A A Þ1

ð5:20aÞ

Substitute all the known values into the set of equations in 5.20a to obtain the number of overall liquid-based transfer units, NTUOL. =

NTU OL

xA2  xA - xA 2 ln

- xA1 - xA - xA

xA - xA xA - xA

1

2 1

0:00041 - 0:000041 = ð0:00041 - 0:000125Þ - ð0:000041 - 0Þ 0:00041 - 0:000125 ln 0:000041 - 0 = 2:93 Calculate the height of an overall liquid phase transfer unit using Eq. 5.18. Z = ðNTU OL ÞðHTU OL Þ ) Z 5:3 m = = 1:81 m HTU OL = NTU OL 2:93 B. Determine the required value of the overall liquid-based capacity mass transfer coefficient using the first part of Eq. 5.22. HTU OL =

L1 L1 ) K xa = ) K x að1 - xA ÞLM ðHTU OL Þð1 - xA ÞLM L1

Kxa = HTU OL

ð1 - xA1 Þ - 1 - xA1 ln

ð1 - xA1 Þ 1 - xA1

10:61

=

kmol min  m2

ð1 - 0:000041Þ - ð1 - 0Þ 1 - 0:000041 ln 1-0 3 = 5:862 kmol= min  m  mole fraction ð1:81 mÞ

5.5

Distillation

5.5

345

Distillation

Distillation is a widely used separation/purification operation in the chemical industry [1, 2]. The components are separated based on the differences in their boiling points/volatilities. When the feed containing a mixture of components is fed to the column, the more volatile component evaporates first and travels toward the top of the column due to its low density and the less volatile component remains in the liquid state and drops down to the bottom of the column due to its higher density. The overhead vapors are condensed and collected as a top product containing high percentage (90+%) of the component with a lower boiling point and the bottom’s liquid containing a significant amount of the higher boiling point component is collected as the bottom product. However, to achieve complete separation, it is necessary to carry out repeated vaporizations and condensations. In practice, a fraction of the condensed overhead vapor is recycled as liquid into the top of the tower. The recycled overhead liquid stream is called as the reflux. Similarly, a portion of the bottom product is vaporized in a still called the reboiler, and the vapor from the reboiler is recirculated through the tower. The reflux from the top and the vapor from the bottom create intimate contact between the bubbling vapor and the falling liquid, promoting exchange of mass and heat, thus enhancing the separation efficiency. Due to the intimate mixing between vapor and liquid, the two phases are very nearly in equilibrium during distillation. For distillation to be practically feasible in separating components from a mixture, the relative volatility between the components, αij,must be greater than 1.25. Greater the relative volatility, easier is the separation process. Relative volatility has been discussed in detail in Sect. 2.5.4 in Chap. 2 on Thermodynamics.

5.5.1

Batch Distillation

In batch distillation, the liquid mixture of components is heated for a specified time period in a still facilitating the vaporization of the more volatile component, which is condensed and collected as the distillate (D). The less volatile component remains in the still as the residual liquid product (W). If F is the initial batch feed in the still, then the Rayleigh equation (Eq. 5.24) can be used for the batch distillation process [1]. xf

dx y - x

F ln = W

ð5:24Þ

xi

If the relative volatility is constant, then Rayleigh’s equation can be integrated to obtain the following result.

346

5

ln

Mass Transfer

F 1 x 1 - xi ln i - α ln = W xf 1 - xf ð α - 1Þ

ð5:25Þ

In Eq. 5.25, xi and xf are the initial and final compositions of the liquid in the still. Further, overall material and component balances result in the following equations. F=W þ D

ð5:26Þ

Fxi = Wxf þ Dyavg

ð5:27Þ

In Eq. 5.27, yavg is the average composition of the distillate. In the preceding equations, composition refers to the composition of the more volatile component. Example 5.6 A mixture of 60 mole% ethanol and 40 mole% water is batch distilled such that 20 mole% of the initial feed remains in the still. Determine A. the composition of the residue in the still B. the average composition of the distillate Solution A. Basis: 100 moles of feed initially in the still. Since 20% of the initial feed remains in the still, )W = 20 moles. From overall material balance (Eq. 5.26), calculate the moles of the distillate produced. F = D þ W ) D = F - W = 100 moles - 20 moles = 80 moles Determine the relative volatility between ethanol and water using vapor pressures at an assumed temperature, say 100 °C. At 100 °C, the vapor pressures of ethanol (e) and water (w) are: sat Psat e = 1190 mm Hg, and Pw = 760 mm Hg

(Source: https://en.wikipedia.org/wiki/Ethanol_(data_page))

αew =

Psat 1190 mm Hg e = = 1:57 760 mm Hg Psat w

Since ethanol is the more volatile component in the feed mixture, apply the integrated form of Rayleigh’s equation (Eq. 5.25) to ethanol. Substitute all the known values, including xi = 0.60, and solve for xf by trial and error using an Excel spreadsheet.

5.5

Distillation

347

ln ln

F 1 x 1 - xi ln i - α ln = ) W xf 1 - xf ð α - 1Þ

100 moles 1 0:60 1 - 0:60 = ln þ ð1:57Þ ln 20 moles xf 1 - xf ð1:57 - 1Þ 0:40 0:60 þ ð1:57Þ ln 1 - xf xf xf = 0:063 ð6:3%Þ

0:917 = ln

)

)

B. Determine the average composition (yavg), of the distillate using Eq. 5.27. Fxi = Wxf þ Dyavg ) Fxi - Wxf ð100 molesÞð0:6Þ - ð20 molesÞð0:063Þ yavg = = 0:7342 = D 80 moles

5.5.2

Continuous Distillation

Continuous distillation is the most common type of distillation used in the industry. Continuous distillation occurs under steady-state conditions, where there is a continuous feed as well as continuous withdrawal of top and bottom products with constant flow rates [1, 2]. The schematic diagram of a typical continuous distillation column including reflux and reboiling is shown in Fig. 5.5. The feed is introduced in the center of the column and the portion of the column above the feed stage is called the Rectifying Section and the portion of the column below the feed stage is called the Stripping Section. The distillation unit shown in Fig. 5.5 has N theoretical or ideal stages. The condenser being a total condenser, (i.e., the overhead vapor is completely condensed) is stage 0 and is not one of the theoretical stages. In this case, the top stage (tray) in the column is stage 1. In contrast, a partial condenser represents a theoretical stage and is counted as stage 1 and the top tray in the column becomes stage 2. The reboiler represents stage (N + 1). Overall material and component balances for the entire column (envelope I-J-K-L, colored brown in Fig. 5.5): F=DþB

ð5:28Þ

FxF = DxD þ BxB

ð5:29Þ

Note: The “component” here refers to the more volatile component.

348

5

Mass Transfer

Fig. 5.5 Distillation schematic and nomenclature

Energy balance for the entire column is FhF þ Q_ R = DhD þ BhB þ Q_ C

ð5:30Þ

In Eq. 5.30, Q_ R is the rate of energy (heat) supplied in the reboiler and Q_ C is the rate of energy (heat) removed in the condenser. Overall material and component balance for the rectifying section (envelope A-B-C-D, colored red in Fig. 5.5): V nþ1 = Ln þ D

ð5:31Þ

V nþ1 ynþ1 = Ln xn þ DxD

ð5:32Þ

Combine Eqs. 5.31 and 5.32 and simplify to obtain the following result.

5.5

Distillation

349

ynþ1 =

Ln D x þ x Ln þ D D Ln þ D n

ð5:33Þ

With the assumption of constant molal overflow under the condition of the molar heats of vaporization being nearly equal for each component, the molar flow rates of the liquid and vapor are nearly constant in each section of the column. Therefore, V nþ1 = V n = V n - 1 = . . . . . . = V and Lnþ1 = Ln = Ln - 1 = . . . . . . = L: The subscripts for L can be dropped from Eq. 5.33. Also, the external reflux ratio, R, is defined as L/D. Divide both numerator and denominator of each term on the right hand side of Eq. 5.33 by D and substitute R for L/D to obtain the following equation. ynþ1 =

R x x þ D Rþ1 n Rþ1

ð5:34Þ

Equation 5.34 represents a straight line, and it is the equation for the operating xD R line for the rectifying section with slope = Rþ 1 and y-intercept = Rþ1. Energy balance for the Rectification Section is V nþ1 hnþ1 = Ln hn þ DhD þ Q_ C

ð5:35Þ

Similar balances can be performed for the stripping section. Start with the overall material balance for the reboiler. L0 = V 0 þ B ) V 0 = L0 - B

ð5:36Þ

Component balance for the stripping section is L0M xM = V 0Mþ1 yMþ1 þ BxB ) yMþ1 =

L0M Bx xM - 0 B V 0Mþ1 V Mþ1

ð5:37Þ

With the assumption of constant molal overflow, the subscripts can be dropped for L′ and V′ resulting in the following equation for the operating line for the stripping section (after combining Eqs. 5.36 and 5.37). yMþ1 =

L0 Bx xM - 0 B L -B L -B 0

ð5:38Þ

350

5

Mass Transfer

Equation 5.38 is a straight-line representation of the operating line for the 0 B stripping section with slope = L0 L- B and y-intercept = - LBx 0 - B. Energy balance for the stripping section is L0M hM þ Q_ R = V 0Mþ1 hMþ1 þ BhB

5.5.2.1

ð5:39Þ

McCabe–Thiele Method for Determining Theoretical Stages

The McCabe–Thiele method for determining the number of theoretical stages for a given separation situation in distillation is a graphical method illustrated in Fig. 5.6. The McCabe–Thiele method involves the following assumptions: • Constant molal overflow, that is, liquid and vapor flows being nearly equal in each section (rectifying and stripping sections) of the column. • Constant relative volatility. • No excessive temperature variations across the column. • Small heat losses from the column. In Fig. 5.6, R line refers to the rectifying section operating line (Eq. 5.34), S line refers to the stripping section operating line (Eq. 5.38), q line refers to the feed line, and Rmin line refers to the rectifying section operating line with minimum reflux ratio.

Fig. 5.6 McCabe–Thiele method for determining the number of theoretical stages

5.5

Distillation

5.5.2.2

351

q Line and Feed Condition

q is a variable that characterizes the condition of the feed being introduced into the column. q is defined as the ratio of the molar enthalpy required to completely vaporize the feed from its existing condition to the normal molar enthalpy of vaporization of the feed in its saturated liquid condition. The mathematical interpretation of q is shown in Eq. 5.40. q=

hv - hfeed hv - hsat:liq

ð5:40Þ

In Eq. 5.40, hv represents the molar enthalpy of the feed in its saturated vapor state. Thus, the value of q depends on the feed condition as described below. 1. Feed is a sub-cooled liquid: In this case, additional heat is required for the sensible heating of the feed from its existing sub-cooled temperature to its normal boiling point. Therefore, Eq. 5.40 takes the following form.

q=

cp ðT b - T feed Þ þ hv - hsat:liq hv - hsat:liq

=1 þ

cp ðT b - T feed Þ hv - hsat:liq

ð5:41Þ

From Eq. 5.41, it is clear that for a sub-cooled liquid, q > 1. 2. Feed is a saturated liquid: In this case, hfeed = hsat:liq . Substitute this into Eq. 5.40.

q=

hv - hsat:liq hv - hsat:liq

= 1:0

ð5:42Þ

If the feed is a saturated liquid, then q = 1.0. Sub-cooled liquid or saturated liquid are the most commonly encountered feed conditions. Other feed conditions are described here for the sake of completeness. 3. Feed is partially vaporized: In this case, it can be shown that, 0 < q < 1. 4. Feed is a saturated vapor: In this case, it can be shown that, q = 0. 5. Feed is a super-heated vapor: In this case, it can be shown that, q < 0. The q line represents the locus of all possible intersection points of the R line and the S line. The q line is a straight line with the equation

352

5

Mass Transfer

Fig. 5.7 q lines for different feed conditions

y=

q x x- F q-1 q-1

ð5:43Þ

Therefore, the slope of the q line is q/(q - 1). When the feed is sub-cooled liquid, q > 1) positive slope (>1) of q line. If the feed is a saturated liquid, q = 1.0 and the slope of the q line is infinity, which implies a vertical q line. If the feed is a saturated vapor, q = 0 and the slope of the q line is zero, which implies a horizontal q line. The orientation of the q line for different feed conditions is shown in Fig. 5.7.

5.5.2.3

Reflux Ratio Range and Optimum Reflux Ratio

As shown in Fig. 5.8, the rectifying line with the minimum reflux ratio passes through the intersection of the equilibrium line and the q line. Since this intersection point is on the equilibrium curve, the driving force for mass transfer (which is usually the difference between the actual mole fraction and the equilibrium mole fraction) is zero. This condition implies that infinite number of stages are required to achieve the desired separation. Therefore, the minimum reflux ratio, Rmin, results in infinite number of theoretical stages. It is called “minimum reflux ratio” because it is not possible to have a reflux ratio less than this minimum value. As the R line pivots about the point (xD, yD = xD), the minimum reflux occurs when the R line passes through the pinch point (infinite stages). When the R line moves down along the q line (rotates in a counterclockwise direction about the pivot point), the separation distance increases between the equilibrium line and the

5.5

Distillation

353

Fig. 5.8 Minimum reflux, total reflux, and in-between

operating R line, resulting in increasing driving force, thereby reducing the number of theoretical stages required. As a limiting case, when the R line superimposes on the y = x line, the driving force is at its maximum possible value, resulting in minimum number of theoretical stages. However, total reflux implies no products. The limiting cases of reflux are Rmin) infinite number of theoretical stages and Rtotal) minimum number of theoretical stages. In practice, a reflux ratio between these extreme values is used. Finding an optimum reflux ratio is based on minimizing the total costs – fixed costs, which increase with number of stages used plus operating costs, which increase in proportion to the value of the reflux ratio. In practice a reflux ratio in the range of 1.2–1.5 is employed. The range of reflux ratios along with the minimum number of stages (at total reflux) are illustrated in Fig. 5.8.

5.5.2.4

Construction of McCabe–Thiele Diagram

The steps involved in the construction of the McCabe–Thiele diagram are as follows: 1. Generate the equilibrium curve for the system by using the given equilibrium data or by using relative volatility equation (Eq. 2.48, y = αx/[1 + (α - 1)x]). 2. Draw the 45° line, that is y = x line. 3. Draw vertical lines at xB, xF, and xD. 4. Draw the q line from (xF, yF = xF) using information about the feed condition.

354

5

Mass Transfer

5. Draw the Rmin line from (xD, yD = xD) through the intersection of the q line with the equilibrium curve (pinch point) and extend it to the y-axis to obtain the yintercept. As shown in Fig. 5.4, the y-intercept of the Rmin line is xD/(Rmin + 1). Since the y-intercept and xD are known, determine Rmin. 6. Calculate Ractual as specified (1.1–1.5 times the minimum reflux) and determine the slope of the actual rectifying line using the following equation. mactual R line =

Ractual Ractual þ 1

ð5:44Þ

7. Construct the actual rectifying line from (xD, yD = xD) using the slope information [m = (y2 - y1)/(x2 - x1) or y2 = mx2], where x2 is any random point on the actual rectifying line. 8. Construct the actual stripping line by joining (xB, yB = xB) and the intersection of the actual rectifying line with the q line (as shown in Fig. 5.4). 9. Step-off the number of stages (as shown in Fig. 5.4).

Example 5.7 50 kmol/min of a feed consisting of 30 mole% n-hexane and 70 mole% n-octane is fed to a continuous distillation column at 350 K and the boiling point of the feed mixture is 385 K. The desired distillation composition is 94 mole% n-hexane and the bottom product should contain 5 mole% n-hexane. A reflux ratio of 1.50 times the minimum value is recommended. The specific heat of the feed is 244 kJ/kmol.K and the enthalpy of vaporization is 34445 kJ/kmol. The equilibrium data (in terms of mole fraction of n-hexane) is xC6 H14 yC6 H14

0 0

0.1 0.3

0.2 0.57

0.3 0.7

0.4 0.78

0.5 0.85

0.6 0.92

Determine: A. B. C. D.

the production rates of the distillate and the bottom product. the minimum number of stages at total reflux. the minimum reflux ratio. the theoretical stages required.

0.7 0.95

1 1

5.5

Distillation

355

Solution A. Draw the schematic diagram for the process as shown.

Overall mass balance (around the red colored envelope): F = B þ D ) 50

kmol kmol = B þ D ) B = 50 -D min min

Component balance for n-hexane, the more volatile component (around the red colored envelope): FxF = DxD þ BxB ) kmol kmol ð0:30Þ = ðDÞð0:94Þ þ 50 50 - D ð0:05Þ ) min min D = 14 kmol=min, and kmol kmol kmol B = 50 - D = 50 - 14 = 36 kmol= min min min min

356

5

Mass Transfer

B. Draw the equilibrium curve, as shown, from the given VLE data. Draw the y = x line. On the x-axis, identify the feed (xF = 0.30), bottom (xB = 0.05), and distillate (xD = 0.94) compositions, respectively, and draw the ordinates (vertical lines) from these locations to intersect the y = x line as shown. The y = x line is the operating line at total reflux, resulting in the minimum number of theoretical stages required. Starting at xD = yD = 0.94, step off the stages by drawing horizontal and vertical lines between the equilibrium curve and the y = x line as shown, resulting in four (4) minimum number of theoretical stages required. C. To determine the minimum reflux ratio, the q line needs to be drawn to facilitate the location of the “pinch point.” Since the feed is a sub-cooled liquid (Tfeed < Tb), first calculate the value of q using Eq. 5.41.

q=1 þ

cp ðT b - T feed Þ hv - hsat:liq

=1 þ

kJ ð385 K - 350 KÞ 244 kmolK = 1:25 kJ 34445 kmol

Calculate the slope of the q line from Eq. 5.43. slope of q‐line =

q 1:25 1:25 Δy =5= = = q - 1 1:25 - 1 0:25 Δx

5.5

Distillation

357

Therefore, from xF = yF = 0.30, move one unit to the right horizontally and five units up vertically on the graph to locate another point on the q line. Draw the q line (colored brown in the figure) through the preceding two points and extend it to locate the intersection of the q line with the equilibrium line and this intersection is the “pinch point” with zero driving force, requiring infinite number of stages for separation. Connect xD = yD = 0.94 to the pinch point and extend the line to the y-axis. This line forms the Rmin line (colored blue in the figure). From the graph, the y-intercept of the Rmin line is 0.68. From Eq. 5.34, 0:94 xD = 0:68 ) = 0:68 ) Rmin þ 1 Rmin þ 1 Rmin = 0:3823 Hence, the minimum reflux ratio is Rmin = 0.3823 D. The actual reflux ratio is 1.50 times the minimum reflux ratio. Therefore, Ractual = ð1:50ÞðRmin Þ = 1:50 × 0:3823 = 0:5734 Calculate the y-intercept of the actual R line, that is Ractual, from Eq. 5.34. 0:94 xD = = 0:5974 ≃ 0:60 Ractual þ 1 0:5734 þ 1 Connect xD = yD = 0.94 with y = 0.60 on the y-axis (x = 0). This line forms the Ractual line (colored blue in the figure). Locate the intersection of the Ractual line with

358

5 Mass Transfer

the q line. Label this intersection point as “Z.” The portion of the Ractual line from xD = yD = 0.94 to the intersection point “Z” forms the operating line for the rectification section (colored blue in the figure). Connect the intersection point “Z” with xB = yB = 0.05. The resulting line (green color in the figure) is the operating line for the stripping section. Step off the stages by drawing horizontal and vertical lines between the equilibrium curve and the operating lines as shown, resulting in eight (8) theoretical stages required. Example 5.8 A distillation column operates with an external reflux ratio of 2.05, which is 1.4 times the minimum reflux. The y-intercept of the Rmin line is measured to be 0.40. The feed consists of 40 mole% A, the more volatile component. The feed condition is saturated liquid. The bottom product consists of 8 mole% of component A in the feed. Assume a feed rate of 100 lbmol/hr. Determine A. B. C. D.

the composition of the distillate. the molar flow rates of the distillate and the bottom product. the slope of the actual R line. the coordinates of the “pinch point.”

Solution Draw the schematic diagram for the process as shown.

A. Calculate Rmin using the values of Ractual, and (Ractual/Rmin) ratio. Rmin =

Ractual 2:05 = = 1:464 1:4 1:4

5.5

Distillation

359

Calculate the composition of the distillate, xD, from Eq. 5.34 using the given value of the y-intercept . xD = 0:40 ) Rmin þ 1 xD = ðRmin þ 1Þð0:40Þ = ð1:464 þ 1Þð0:40Þ = 0:986 B. Determine the molar flow rates of the distillate and bottom product using overall mass balance and component balance for the more volatile component, A. Overall mass balance (around the red colored envelope): F = B þ D ) 100

lbmol lbmol = B þ D ) B = 100 -D hr hr

Component balance for the more volatile component (around the red colored envelope): FxF = DxD þ BxB )

100

= ðDÞð0:986Þ þ 100

lbmol ð0:40Þ hr

lbmol - D ð0:08Þ ) hr

D = 35:32 lbmol=hr, and lbmol lbmol lbmol B = 100 - 35:32 = 64:68 lbmol=hr - D = 100 hr hr hr C. Equation 5.34, which is the equation for the Ractual line, is reproduced here for reference. ynþ1 =

R x x þ D Rþ1 n Rþ1

From the preceding equation, the slope of the Ractual line is SlopeRactual =

2:05 Ractual = = 0:6721 Ractual þ 1 2:05 þ 1

D. From Fig. 5.6, the Rmin line passes through the “pinch point.” Similar to the slope of the Ractual line, the slope of the Rmin line from Eq. 5.34 is SlopeRmin =

1:464 Rmin = = 0:5942 Rmin þ 1 1:464 þ 1

360

5

Mass Transfer

Using the slope-intercept form of equation for a straight line y = mx + c, the equation for operating line of the Rmin line is, y = 0.5942x + 0.40. From Fig. 5.6, the q line passes through the “pinch point” and from Fig. 5.7, the q line is vertical for a feed in saturated liquid state. The preceding concepts are illustrated in the following diagram. From the illustration, it is clear that the x-coordinate of the pinch point is the same as that of the feed, that is, xP = xF = 0.40. Further, since the “pinch point” also lies on the Rmin line, the coordinates of the pinch point must satisfy the equation for the Rmin line. Therefore,

y = 0:5942x þ 0:40 ) yP = 0:5942xP þ 0:40 ) yP = ð0:5942Þð0:40Þ þ 0:40 = 0:64 Hence, the coordinates of the “pinch point” in this case are: xP = 0:40 and yP = 0:64

5.5.2.5

Determining Minimum Number of Theoretical Stages Using Fenske Equation

The minimum number of theoretical stages for a binary system can be analytically calculated using the Fenske equation [1, 5]. The minimum number of stages calculated by Fenske’s equation includes any theoretical stages represented by condenser and reboiler. The Fenske equation is

5.6

Psychrometrics and Psychrometric Chart

N min =

ln

361 1 - xB xB

xD 1 - xD

ð5:45Þ

ln α

Example 5.9 Analytically, calculate the minimum number of theoretical stages using the data in Example 5.7. Solution The following data relevant to this problem is available from Example 5.7: xD = 0:94, xB = 0:05, and from eqbm data, when x = 0:50, y = 0:85 First, determine the relative volatility using equilibrium data and Eq. 2.48 (from the thermodynamics chapter). Equation 2.48 is, y=

αx 0:5α ) 0:85 = ) α = 5:67 1 þ ðα - 1Þx 1 þ 0:5ðα - 1Þ

Substitute all the known values into Fenske’s equation (Eq. 5.45) to determine the minimum number of theoretical stages required.

N min =

ln

xD 1 - xD

ln α

1 - xB xB

=

ln

0:94 1 - 0:94

1 - 0:05 0:05

ln 5:67

= 3:28

The preceding result is in good agreement with (3+) minimum theoretical stages required from the graphics based McCabe–Thiele method.

5.6

Psychrometrics and Psychrometric Chart

Psychrometrics is the study of air–water vapor mixtures (moist air). In chemical engineering, psychrometrics has important applications in equipment such as cooling towers and dryers. In drying operation, moisture is removed from a wet solid. This involves heat transfer to vaporize the moisture and subsequent mass transfer of water vapor to surrounding air. Hence, most psychrometric operations involve “simultaneous heat and mass transfer (SHMT).”

5.6.1

Moist Air Properties and Definitions

A good understanding of psychrometrics requires knowledge of the properties of moist air. The definitions of important properties of moist air are given here [4].

362

5

Mass Transfer

Dry Bulb Temperature (TDB): The dry bulb temperature is the ordinary temperature of air measured by a regular thermometer. Wet Bulb Temperature (TWB): The wet bulb temperature is the temperature measured by a wet bulb thermometer. As the name implies, in a wet bulb thermometer the thermometer bulb is surrounded by a moist wick. Depending on the condition of the surrounding air, moisture from the wick will evaporate into air, using the internal energy of the thermometer itself, thus indicating a temperature lower than the dry bulb temperature. Dew Point Temperature (TDP): Some of the water vapor in air will condense when the air–water vapor mixture is cooled. The temperature at which condensation begins is called dew point temperature. Note: In general, TDB > TWB > TDP and at saturation conditions, that is at 100% relative humidity, TDB = TWB = TDP. Humidity Ratio (ω): Humidity ratio quantifies the moisture content of air on a mass basis. It is the ratio of the mass of water vapor to the mass of dry air. The mathematical equation for humidity ratio is ω=

lbm water vapor kg water vapor = kg dry air lbm dry air

ð5:46Þ

Since specific humidity is dimensionless, it will have the same numerical value in all systems of units. Relative Humidity (ϕ or RH): Relative humidity is a measure of how close the air is to the saturated state. Air can hold a limited amount of moisture at any given temperature. On nights when there is a rapid drop in temperature, the air tends to become supersaturated since the amount of moisture air can hold will decrease with decrease in temperature. Supersaturated air has excess moisture which condenses on automobile surfaces. The condensate can be noticed after a cool night. Fully saturated air has 100% relative humidity. By definition, relative humidity (rh) is the ratio of the partial pressure of water vapor in the given mixture to the partial pressure of water vapor if the mixture were to be saturated at the mixture temperature. The saturation partial pressure can be obtained from steam tables, and it is the saturation pressure at the mixture temperature. The formula for relative humidity is ϕ=

Pw Psat at T DB of mixture

ð5:47Þ

5.6

Psychrometrics and Psychrometric Chart

5.6.2

363

Finding Moist Air Properties from Psychrometric Chart

The properties of moist air can be determined from the Psychrometric Chart [4]. Figure 5.9 illustrates the general layout and the parameters used in the psychrometric chart. The state point of moist air can be located on the chart by using two parameters, for example, dry bulb temperature and relative humidity. Once the state point is located, all the properties of moist air can be determined [4] as shown in Fig. 5.9. The psychrometric chart for USCS units is shown Fig. 5.10 and the psychrometric chart for SI units is shown Fig. 5.11. The following example illustrates the use of psychrometric chart to find the properties of moist air. Example 5.10 Air is at 70 °F and 60% relative humidity. Determine: A. B. C. D. E.

the humidity ratio. the wet bulb temperature. the dew point temperature. the enthalpy per pound mass dry air. the specific volume.

Fig. 5.9 Determining properties of moist air from the psychrometric chart

364

5

Mass Transfer

Fig. 5.10 Psychrometric chart. (USCS units, Generated by the author, N. S. Nandagopal, P E)

Fig. 5.11 Psychrometric chart. (S I units, Generated by the author, N. S. Nandagopal, P E)

5.7

Cooling Towers

365

Solution Locate the state point of moist air at the intersection of 70 °F dry bulb vertical line and 60% relative humidity curve as shown in the figure. Using the state point as a reference, determine all the properties of moist air as shown.

A. B. C. D. E.

5.7

ω = 0.0095 lbm H2O/lbm dry air TWB = 61 °F TDP = 56 °F h = 27.5 Btu/lbm dry air v = 13.55 ft3/lbm dry air

Cooling Towers

In power plants and chemical processing plants, hot water from condensers and heat exchangers needs to be cooled before recirculating back to the exchangers in a closed loop. Typically, this is done in a cooling tower. A cooling tower operates adiabatically [1, 4] since it uses the heat from hot water to humidify the air flowing through the tower in a direction opposite to the hot water spray. Due to the supply of enthalpy of vaporization from the hot water to humidify the air, the hot water cools down. Since some hot water evaporates into the air stream, makeup water must be supplied to maintain constant circulation through the condensers and heat exchangers. The amount of air required to cool the hot water and the quantity of make-up water required can be determined by using mass and energy balances for the cooling tower as shown here. Figure 5.12 is a schematic of a cooling tower with all the relevant variables Note that ‘1’ represents the top of the tower, where hot water enters and ‘2’ represents the bottom of the tower where air enters. Subscript ‘w’ represents water and subscript ‘a’ represents air. Mass balance for water around the envelope (red-dashed lines) shown in Fig. 5.12 results in the following equations.

366

5

Mass Transfer

Fig. 5.12 Schematic for a cooling tower

Mass flow of water in = mass flow of water out m_ w1 þ m_ a ω2 = m_ w2 þ m_ a ω1 m_ w1 - m_ w2 = m_ a ðω1 - ω2 Þ The left-hand side of the preceding equation also represents the quantity of makeup water required. m_ m = m_ w1 - m_ w2 = m_ a ðω1 - ω2 Þ

ð5:48Þ

Energy balance around the envelope results in the following equations. Rate of energy in = Rate of energy out m_ w1 hw1 þ m_ a ha2 = m_ w2 hw2 þ m_ a ha1

ð5:49Þ

Equations 5.48 and 5.49 can be used simultaneously to solve problems on cooling towers as shown in Example 5.11. Example 5.11 8000 kg/hr of water at 40 °C is to be cooled to a temperature of 25 °C by using a cooling tower. Air enters the cooling tower at 20 °C dry bulb and 50% relative humidity and leaves the tower at 32 °C dry bulb and 70% relative humidity. Determine: A. the quantity of air required in terms of standard cubic meters per minute (SCMM). B. the amount of make-up water required in liters per minute.

5.7

Cooling Towers

367

Solution A. Use the same nomenclature as in Fig. 5.10. Locate the state points of moist air on the psychrometric chart and obtain the corresponding state properties as shown. At state point 1 (32 C, DB and 70% rh): wω1 = 0.022 kg H2O / kg d a, and ha1 = 88 kJ / kg d a, and at state point 2 (20 C, DB and 50% rh): wω2 = 0.0075 kg H2O / kg d a, ha2 = 39 kJ / kg d a, and v2 = 0.84 m3 / kg d a

Determine the enthalpies of water from the steam tables (Source: https://www. spiraxsarco.com/resources-and-design-tools/steam-tables/sub-saturated-waterregion#article-top) hw1 = hf at 40∘ C = 167:53 kJ=kg water and hw2 = hf at 25∘ C = 104:83 kJ=kg water Express m_ w2 in terms of m_ w1 using Eq. 5.48, substitute the result into Eq. 5.49, and then solve for m_ a , the mass flow rate of dry air. Subsequently, substitute the known values to obtain the mass flow rate of dry air.

368

5

Mass Transfer

m_ w1 ðhw1 - hw2 Þ ha1 - ha2 - hw2 ðω1 - ω2 Þ kg water kJ kJ 8000 167:53 - 104:83 hr kg water kg water = kg water 0:022 kg d a kJ kJ kJ 88 - 39 - 104:83 kg d a kg d a kg water kg water - 0:0075 kg d a = 10564 kg d a=hr

m_ a =

Calculate the volume flow rate of air at inlet conditions using the specific volume at inlet. kg d a V_ air,in = m_ a ν2 = 10564 hr

0:84

m3 = 8874 m3 =hr kg d a

The air inlet temperature is 20 °C (293 K), whereas the reference temperature for standard cubic meter is 15 °C (288 K). Volume is directly proportional to absolute temperature. Calculate the standard cubic meter per minute (SCMM) of air required using the preceding concepts. 1 hr m3 V_ air req: = 8874 × hr 60 min

288 K = 145:37 SCMM 293 K

B. Calculate the quantity of make-up water required using Eq. 5.48 and substituting the known values. m_ m = m_ a ðω1 - ω2 Þ kg da kg water kg water = 10564 0:022 - 0:0075 kg d a hr kg d a = 153 kg water=hr Calculate the amount of make-up water required in liters per minute. Use the standard density of water, 1 kg/L. m_ m = 153

1 kg water 1 hr = 2:55 L= min × × hr 60 min 1 kg L

5.8

Drying

5.8

369

Drying

Drying is a unit operation where moisture is removed from materials, usually solids. A simple, everyday example is the drying of wet clothes. The wet clothes can be dried through a natural process of exposing the wet clothes to outside sun and air. Heat is transferred to the wet clothes by radiation from the sun as well as by convection from the warm surrounding air. The heat evaporates the moisture from the wet clothes, which is a mass transfer process. Thus, drying is a simultaneous heat and mass transfer (SHMT) process [2, 3]. The wet clothes can also be dried using a clothes dryer, which typically has an electrical element to heat up air and the circulation of clothes and hot air results in drying of the clothes. The clothes dryer can be considered as a batch drying process. However, industrial drying on a large scale typically uses a continuous drying process. Tray dryer is an example of a commonly used continuous drying process. Here, the wet material is continuously fed to a drying area where it comes in contact with hot air.

5.8.1

Drying Mechanism and Rate of Drying

Drying rate [2, 3] is defined as the amount of moisture removed from the wet substrate per unit surface area per unit time. The drying rate is represented by the following symbol: R kg H2 O=m2  s or lbm H2 O=hr‐ft2 A few simple observations related to the drying of clothes in natural sunlight and air leads to important insights and conclusions on the mechanism of drying and consequently the rate of drying. The more the clothes are spread, the faster they dry: This implies that the drying rate is directly proportional to the surface area of drying. Clothes dry faster on hot days: The conclusion here is that heat transfer plays an important role in the drying process. The drying rate is directly proportional to the heat transfer rate from the surrounding entity to the wet material simply because of faster evaporation of water from the wet material. Clothes dry faster on less humid days: The important conclusion here is that the rate of mass transfer of water vapor from the drying surface to the surrounding air is proportional to the difference in concentration (partial pressure in case of gases) difference of water vapor. Air with lower humidity has lower partial pressure of water vapor in the air–water vapor mixture, thus increasing the driving force for mass transfer. In fact, the driving force reaches a limiting value of zero when the surrounding air is saturated (100% relative humidity). Clothes take forever to dry in humid climates.

370

5

Mass Transfer

Clothes dry faster on windy days: The important conclusion here is that the drying rate is proportional to the heat and mass transfer coefficient, both of which will be higher at higher velocities, that is, at higher Reynolds numbers. However, drying rates follow a more complex pattern in the drying of thick, wet matter. Typically, there are three distinct periods [2, 3] in the drying rates – Constant Rate Period, First Falling Rate Period, and Second Falling Rate Period. In practice, hardly any moisture is removed during the second falling rate period and the simplified drying rate model consists of one constant rate period and one falling rate period. Understanding drying rates requires the understanding the definitions different of moisture contents at different stages of the drying process. Moisture content is typically represented by, X, ðkg moisture=kg dry solid or lbm moisture=lbm dry solidÞ: Initial moisture content, Xi, is the moisture content of the wet solid at the beginning of the drying process. Final moisture content, Xf, is the desired moisture content of the wet solid at the end of the drying process. Equilibrium moisture content, Xe, is the limiting moisture content beyond which the material cannot be dried. At this point, the moisture in the solid is in equilibrium with the moisture in the surrounding air. Therefore, the equilibrium moisture content depends on the conditions of the surrounding air. Critical moisture content, Xc, is the moisture content that marks the transition from constant rate drying period to falling rate period. At this point, the moisture content at the surface of the solid is not sufficient to maintain a constant rate of drying. The constant rate period is characterized by the availability of a constant film of water on the drying surface and it is akin to water evaporating from a pool or a container. There is continuous replenishment of water on the drying surface to keep pace with the rate of evaporation. The drying rate starts to fall and continues to decrease due to non-availability of sufficient water on the drying surface, making it necessary for the water to travel from the inner layers to the drying surface. This process takes time and it results in the continuous decrease of the drying rate as water molecules diffuse from deeper layers of the wet solid. The situation just described characterizes the falling rate period. Slowly, and eventually, the wet solid attains the equilibrium moisture, at which stage there can be no further drying. The drying rate falls to zero at equilibrium moisture content. The simplified drying rate model is shown in Fig. 5.13.

5.8

Drying

371

Fig. 5.13 Drying rates

5.8.1.1

Determining the Time Required for Drying

If tc is the time required to reach the critical moisture content, Xc, from the initial moisture content, Xi, (when t = 0), then the constant drying rate is the change in the moisture content per unit area divided by the time required. Since moisture content is expressed in terms of mass of water per unit mass of dry solid, the difference in moisture content has to be multiplied by the mass of the dry solid, (ms), to obtain the amount of water removed during any period of time. The drying rate is simply the amount of water removed per unit area of the drying surface divided by the time taken to remove this water. Based on the preceding concept, the following mathematical expression [2, 3] can be written for the constant drying rate, Rc. and subsequently solved for the constant rate drying time period. Rc =

m s ðX i - X c Þ At c

ð5:50aÞ

tc =

ms ðX i - X c Þ ARc

ð5:50bÞ

In Eqs. 5.50a and 5.50b, ms is the mass of the dry solid and A is the area of the drying surface. It is important to note that the mass of the dry solid remains constant throughout any drying operation. The falling rate period can be analyzed as follows. The driving force during the falling rate period is the difference in the moisture content, X - Xe, where X is any moisture content along the falling rate line and Xe is the equilibrium moisture content. Since the drying rate is proportional to the driving force, the following equation can be written for the drying rate in the falling rate period by using a proportionality constant, K.

372

5

Mass Transfer

R = K ðX- X e Þ

ð5:51Þ

The proportionality constant in Eq. 5.51 is the slope of the falling rate line, which can be obtained from Fig. 5.13 as follows. Note that the drying rate is zero at the equilibrium moisture content.

ð5:52Þ

In Eq. 5.52, Rc is the drying rate at the critical moisture content, Xc, and Re is the drying rate at the equilibrium moisture content, Xe. However, by definition, the drying rate at any time is R=

ms dX A dt

ð5:53Þ

Combine Eqs. 5.51, 5.52, and 5.53 and integrate the resulting equation. tf

ms Rc ms dX Rc dX ðX - X e Þ ) = dt ) = A dt Xc - Xe A ðX - X e Þ Xc - Xe =

ms A

Xc - Xe Rc

dt tc

Xf

dX ) ðX - X e Þ Xc

t total = t c þ

ms A

X - Xe Xc - Xe ln c Rc Xf - Xe

Substitute for tc from Eq. 5.50b to obtain the total time required to reach the final moisture content, Xf, from the initial moisture content, Xi. t total =

ms ðX i - X c Þ ms þ ARc A

t total =

ms ARc

Xc - Xe X - Xe ln c Rc Xf - Xe

ðX i- X c Þ þ ðX c- X e Þ ln

Xc - Xe Xf - Xe

) ð5:54Þ

Example 5.12 A silica brick mix, having a typical critical moisture content of 12%, is dried from 50% moisture content to 5% moisture content. This process typically takes 8 h. The equilibrium moisture content is 2%. Determine

5.8

Drying

373

A. the time periods of constant rate drying and falling rate drying. B. the constant drying rate assuming 100 kg of dry solids and 4 m2 drying surface area. Solution From the given data, the following information about moisture contents (all expressed in terms of kg H2O/kg dry solid) at various stages of the drying process, and the total drying time required can be obtained: X i = 0:50, X c = 0:12, X f = 0:05, X e = 0:02, and t total = 8 hrs By substituting all the known values into Eq. 5.54, determine the quantity

ms ARc

,

where ms is the mass of the dry solid (kg), A is the solid surface area available for drying, and Rc is the drying rate during the constant rate drying rate (kg H2O removed/unit area. unit time). Xc - Xe Xf - Xe

t total =

ms ARc

ðX i - X c Þ þ ðX c - X e Þ ln

8 hrs =

ms ARc

ð0:50 - 0:12Þ þ ð0:12 - 0:02Þ ln

=

)

0:12 - 0:02 0:12 - 0:05

ms m ½0:4157 ) s = 19:24 hrs ARc ARc

A. Determine the constant rate drying period using Eq. 5.50b. m s ðX i - X c Þ ms = ðX i - X c Þ ) ARc ARc t c = ð19:24 hrsÞð0:50 - 0:12Þ = 7:31 hrs tc =

Calculate the falling rate drying period by subtracting the constant rate period from the total time required for drying. t falling = t total - t c = 8 hrs - 7:31 hrs = 0:69 hrs B. Calculate the constant drying rate using Eq. 5.50a. Rc =

m s ðX i - X c Þ At c kg H2 O kg H2 O - 0:12 kg dry solid kg dry solid ð4 m2 Þð7:31 hrsÞ

ð100 kg dry solidÞ 0:50 = = 1:30 kg H2 O=m2  hr

374

5.8.2

5

Mass Transfer

Adiabatic Drying and Adiabatic Saturation Process

A counterflow, continuous dryer, uses relatively dry, preheated ambient air flowing in a direction opposite to flow of wet product to remove moisture. The heat required for vaporization of moisture from the wet solid comes from the sensible cooling of the hot air stream. The evaporation of water into the air stream makes the air stream leaving the dryer more humid. The entire drying process is adiabatic, and it is called adiabatic saturation of air. An adiabatic drying process based on adiabatic saturation, can be represented by a constant wet bulb temperature line on a psychrometric chart. Example 5.13 It is proposed to reduce the moisture content in a wet solid from 20% water, on a wet basis, to 5% water by using hot air in a continuous tray dryer. The feed rate of the wet solid is 485 lbm/hr. Ambient air at 60 °F and 30% relative humidity is preheated to 100 °F and then fed to the dryer. After adiabatic saturation, air leaves the dryer with 75% relative humidity. Determine: A. the volume flow rate of air required. B. the kW rating of the heating coil required in the preheater if the preheater has an efficiency of 90%. Solution A schematic diagram for the process is drawn as shown.

A. The entering solid has 20% moisture content on a wet basis. Using this information and the given mass flow rate, calculate the mass flow rate of bone-dry solid. lbm wet solid 0:8 lbm bone dry solid lbm wet solid hr = 388 lbm bone dry solid=hr

m_ bds = 485

The partially dried solid coming out of the dryer has a moisture content of 5%. Based on this information, calculate the mass flow rate of the partially dry solid.

5.8

Drying

375

lbm bone dry solid 1 lbm partially dry solid hr 0:95 lbm bone dry solid = 408 lbm partially dry solid=hr

m_ pds = 388

m_ a is the mass flow rate of dry air, which remains constant throughout the processes in the pre-heater and dryer. The increase in moisture content of air due to the drying process is m_ a ðω4 - ω3 Þ, where ω3 and ω4 are moisture contents in air before and after the drying process. To determine ω4 and ω3 and , use the psychrometric chart. Locate the state points of air on the psychrometric chart as shown. Calculate the mass of water evaporated and absorbed by air by mass balance for water in the solids. mass of water vapor absorbed by air = mass of water evaporated from wet solid = mass of wet solid in  mass of partially dry solid out m_ a ðω4 - ω3 Þ = m_ ws - m_ pds = 485 ) m_ a =

lbm lbm - 408 = 77 lbm H2 O=hr hr hr

77 lbm H2 O=hr ðeqn:for mass flow rate of dry air reqd:Þ ω 4 - ω3

State point 1(60 °F, 30% rh): ω1 = 0:0035 lbm H2 O=lbm dry air, v1 = 13:3 ft3 =lbm d a State point 2 and 3 (100 °F, ω1 = ω2 = ω3, sensible heating at const. moist.): ω1 = ω2 = ω3 = 0:0035 lbm H2 O=lbm dry air To locate state point 4, move along the constant wet bulb temperature line (because of adiabatic saturation of air) to the final relative humidity of 75%. At state point 4, ω4 = 0.011 lbm H2O/lbm dry air. Substitute the preceding values into the equation for the required mass flow rate of dry air derived earlier.

376

5

m_ a

77 lbm H2 O=hr = = ω 4 - ω3 = 10, 267 lbm d a=hr

Mass Transfer

lbm H2 O hr lbm H2 O lbm H2 O - 0:0035 0:011 lbm d a lbm d a 77

Calculate the volume flow rate of air required using the specific volume at state 1, v1. V_ a1

= m_ a v1 = 10267

lbm d a hr

13:3

ft3 lbm d a

= 1:3655 × 105 ft3 =hr B. Determine the enthalpies of air at state points 1 and 2, that is, h1 and h2, from the psychrometric chart. h1 = 18 Btu=lbm d a, and, h2 = 28 Btu=lbm d a If Q_ in , is the rate of heat input in the pre-heater, energy balance for the preheater results in the following equations. Energy in = energy out m_ a h1 þ Q_ in = m_ a h2 ) lbm d a Q_ in = m_ a ðh2 - h1 Þ = 10267 hr = 102670 Btu=hr

28

Btu Btu - 18 lbm d a lbm d a

Convert the heat input rate in the pre-heater to kW and also factor in the efficiency of the pre-heater. Q_ in =

5.9

102670 Btu hr × 0:90

1 kW 3412Btu hr

= 33:43 kW

Liquid–Liquid Extraction

Liquid–liquid extraction (LLE) is a separation process where a solute is extracted from a feed solution by using a liquid solvent [1, 5]. The process takes place completely in the liquid phase and all the components in the process are liquids. The liquid containing the solute, the desired product, is the feed solution. The liquid that preferentially dissolves the solute is called the solvent. The feed and the solvent are thoroughly mixed resulting in the solute migrating to the solvent. After the

5.9

Liquid–Liquid Extraction

377

Fig. 5.14 Single-stage extraction process

mixing process, the resulting mixture is allowed to settle down, which eventually results in two immiscible liquid layers, which can then be separated based on density difference. One of the immiscible liquids is called the extract and the other immiscible liquid phase is called the raffinate. The extract liquid phase is rich in the solvent and primarily consists of the solute extracted by the solvent. The raffinate liquid phase is lean in the solvent used for extraction of the solute and contains primarily the other component of the feed solution, (called the ‘career’), traces of the solvent, and the solute to be extracted. Consider an organic solution of benzene and acetic acid, which constitutes the feed solution in the extraction process. Water is used as the solvent to dissolve the acetic acid and usually a pure solvent is used. During thorough mixing of the benzene-acetic acid feed and water solvent, acetic acid distributes itself between the benzene and water phases, more preferentially toward the water phase since acetic acid has high solubility in water. After allowing the mixture to settle down, two distinct liquid phases are formed – a water-rich phase containing more acetic acid and a benzene-rich phase containing traces of the original acid and perhaps some traces of water as well. The water-rich phase is the extract, and the benzenerich phase is the raffinate. Due to higher density, the water rich phase settles down at the bottom of the mixer-settler unit and acetic acid is separated from the water-rich phase using methods such as azeotropic distillation. More and more acetic acid can be removed from the benzene feed solution by repeated stages of the extraction operation. A combination of mixer and settler constitutes a single stage of the extraction process. A schematic diagram of a single-stage extraction process is shown in Fig. 5.14. Species “A” in the feed solution is also known as the ‘career’ and species “C” is the solute. Species “B” is the solvent. Here is a summary of the terminologies [1, 5] along with references to the benzene, acetic acid, water system. Feed solution: Liquid mixture of career (benzene-A) and solute (acetic acid-C). Extract: Liquid mixture of solute (acetic acid-C) and solvent (water-B). Raffinate: Liquid mixture of career (benzene-A) and traces of solute (acetic acid-C). Other examples of application liquid–liquid extraction are: • Extraction of penicillin from fermentation mixture using butyl acetate as solvent. • Extraction of acetone from a water-acetone solution using chloroform as solvent. • Extraction of long-chain fatty acids from vegetable oils using liquid propane as solvent. • Extraction of aromatic organic components from lube oil fractions using furfural as solvent.

378

5.9.1

5

Mass Transfer

Ternary Phase Diagrams

As shown in Fig. 5.14, there are three (3) major components/species in the extraction process – A, B, and C. This necessitates the use of Triangular (ternary) Composition and Phase Diagrams. An example of a ternary diagram is shown in Fig. 5.15. Each apex in the triangle represents 100% of the species indicated at the apex. Any point in the ternary phase diagram defines a mixture of the three species with a particular composition. As examples, reference point 1 has a composition of 20% A, 50% B, and 30% C and reference point 2 has composition 60% A, 30% B, and 10% C. At any given temperature (isotherm), there will be an equilibrium miscibility boundary, within which the two separate immiscible phases will exist in equilibrium. The miscibility boundary represents the fusion of the solubility curves for the two different immiscible liquid phases. Any mixture of the three components outside the miscibility boundary will be a single, homogeneous liquid phase. The details of the miscibility boundary and tie-lines (explained later) are shown in Fig. 5.16. In Fig. 5.16, the area enclosed by the equilibrium solubility curve (miscibility boundary), DRPEK represents the two-phase region. The solubility curve is a confluence of two curves, DRP on the left side and PEK on the right side. DRP represents the equilibrium concentrations of raffinate portions and PEK represents the equilibrium concentrations of extract portions of the two-phase liquid mixtures. Point M represents a mixture obtained by mixing certain quantities of feed solution and pure solvent and the line through point M connecting points R and E (the equilibrium concentrations of the raffinate and extract) is called as the tie line. The point P represents the junction of equilibrium concentrations of the raffinate and extract phases and as such, at this point, also known as the plait point, both the raffinate and extract will have the same composition and no further separation is possible at the “plait point.” Fig. 5.15 Example of ternary phase diagram

5.9

Liquid–Liquid Extraction

379

Fig. 5.16 Ternary phase diagram and miscibility boundary

5.9.2

Lever Rule

The amount of feed and solvent required to form a mixture characterized by M in Fig. 5.16 can be obtained by using overall and component material balances or alternatively by using the lever rule, also known as the inverse lever arm rule. The same approach can be used in determining the amounts of raffinate and extract phase formed. Consider the tie line, RME. The mixture point M, divides the tie line into two arms, RM and ME. The lever rule states that “The mixture point in a tie line divides the tie line into two lever arms. The fraction of a particular phase in a two phase mixture is equal to the length of the opposite lever arm divided by the total length of the tie line.” Therefore, using the lever rule, the mass fractions of the raffinate and extract in the mixture, M, can be calculated by the following equations. xR =

ME opposite lever arm = RE total length of tie‐line

ð5:55Þ

xE =

opposite lever arm RM = total length of tie‐line RE

ð5:56Þ

The details of using material balances and lever rule for a mixture of a feed and solvent are illustrated in Example 5.14. Example 5.14 Consider the mixture represented by point “M” in Fig. 5.16. Assuming use of pure solvent and 300 kg of the mixture, determine:

380

A. B. C. D.

5

Mass Transfer

the amount and composition of the feed solution. the amount of solvent used. the amount and composition of the extract. the amount and composition of the raffinate.

Solution Since pure solvent is used, start the mixture line at point B in Fig. 5.16 and extend the line through the mixture point M to the opposite base AC, which represents the binary mixture of feed solution consisting of A (career) and C (solute). The intersection of the mixture line with the base AC, represents the composition of the feed solution as shown in the figure. A. From the figure, the composition of the feed is 59% C (solute) and 41% A (career).

Consider the line segment BMF. The point M on the line segment BMF represents the mixture of feed and solvent. The measurements of the lever arms FM and MB are indicated on the phase diagram and accordingly, FM = 20 units MB = 60 units. Using the lever rule, amount of feed MB 60 = = = 0:75 ) FB 60 þ 20 amount of mixture F = 0:75M = 0:75 × 300 kg = 225 kg B. From mass balance for the mixture, the amount of solvent used is B = M - F = 300 kg - 225 kg = 75 kg C. Consider the line segment RME. The point M on the line segment RME represents the mixture of raffinate and extract. The measurements of the lever arms RM and ME are indicated on the phase diagram and accordingly, RM = 30 units ME = 14 units. Using the lever rule,

5.9

Liquid–Liquid Extraction

381

amount of extract RM 30 = = = 0:682 ) amount of mixture RE 30 þ 14 E = 0:682M = 0:682 × 300 kg = 204:6 kg The composition of the extract can be obtained by determining the coordinates of point E on the ternary phase diagram. Accordingly, E  51%C, 15%A, and 34%B D. Using the lever rule, amount of raffinate ME 14 = = = 0:318 ) amount of mixture RE 30 þ 14 R = 0:318M = 0:318 × 300 kg = 95:4 kg The composition of the raffinate can be obtained by determining the coordinates of point R on the ternary phase diagram. Accordingly, R  24%C, 70%A, and 6%B Alternate Solution Using Material Balances: This problem can also be solved using mass balances (overall and component balances). As an example, the calculation of the amount of feed, F, is shown here. Component balance for the solute, C, is FxCF þ BxCB = MxCM ) F ð0:59Þ þ Bð0Þ = 300 × :45 ) F = 229 kg The value for F obtained by mass balance compares very well with the value for F obtained by using the lever rule. The slight discrepancy is due to round off errors while reading the ternary graph.

5.9.3

Multistage Extractions

A two-stage counterflow extraction process is shown in Fig. 5.17. The analysis and calculations related to the two-stage extraction process can be done using mass balances, and lever rule as shown in Example 5.15.

382

5

Mass Transfer

Fig. 5.17 Two-stage counterflow extraction process

Example 5.15 In the equilibrium ternary diagram shown below, species C is the solute to be extracted using solvent B from a feed solution containing A (career species) and C.

The data applicable to a two-stage extraction process is shown in the schematic diagram below. Note that the variable ‘S’ is used for the solvent and it can be interchanged with the variable ‘B’ used in the ternary phase diagram.

Determine the flows and compositions of the streams E1 and R2. Solution Obtain the composition of the mixture of the feed and the solvent by mass balances and then locate the mixture point, M, on the phase diagram as shown. Note that point M will be on the line joining the feed and the solvent. Overall mass balance: F þ S = M ) 300 kg þ 180 kg = 480 kg = M Component balance for solute, C:

5.9

Liquid–Liquid Extraction

383

FxF,C þ SyS,C = MxM,C ) ð300 kgÞð0:40Þ þ ð180 kgÞð0Þ = ð480 kgÞðxM,C Þ ) xM,C = 0:25

Locate yE1, C (= 0.35), the composition of the solute in the extract, on the equilibrium curve. This will fix the location of E1 on the phase diagram. Draw a line connecting E1 and M, then extend the line to the other side of the equilibrium curve. The intersection of the extension of line E1M with the equilibrium curve is the location of the raffinate, R2, leaving stage 2. At this point, the composition of R2 is xR2 ,C = 0:14. Note that the line segment R2E1 is not a tie line.

Perform balances around the boundary (red-dashed lines) indicated on the diagram. Overall mass balance: F þ S = E1 þ R2 ) E1 þ R2 = 300 kg þ 180 kg = 480 kg Component balance for solute, C: FxF,C þ SySC = E1 yE1 ,C þ R2 xR2 ,C and R2 = 480 kg - E1 ) ð300 kgÞð0:40Þ þ ð180 kgÞð0Þ = ðE1 Þð0:35Þ þ ð480 kg - E1 ÞðxR2 ,C Þ ) 120 kg = 0:35E1 þ 480xR2 ,C - E1 xR2 ,C E1 = 251:4 kg

xR2 ,C = 0:14

→

R2 = 480 kg - E1 = 480 kg - 251:4 kg = 228:6 kg

384

5

Mass Transfer

Fig. 5.18 Counterflow multistage extraction

Summary of results: E1 = 251:4 kg, yE1,C = 0:35, yE1,A = 0:09, yE1,B = 0:56 R2 = 228:6 kg, xR2,C = 0:14, xR2,A = 0:74, xR2,B = 0:12

5.9.3.1

Finding the Number of Transfer Units Required in Counterflow Multistage Extraction

The schematic diagram for a counterflow multistage extraction system is shown in Fig. 5.18. Analogous to the Eqs. 5.20 and 5.21, used in calculating the number of transfer units (NTU) for gas absorption, the number of transfer units required for liquid extraction can be determined from the following equation [1, 5]. NTU =

xF - xRN ðxF - xF Þ - ðxRN - xRN Þ ln

ð5:57Þ

xF - x F xRN - x RN

In Eq. 5.57, xF is the raffinate composition in equilibrium with the extract composition y1, and xRN is the raffinate composition in equilibrium with the solvent composition ys. Example 5.16 Consider the extraction of ethylene glycol (E) from 165 kg/hr of a feed solution containing 65% (by mass) of ethylene glycol and 35% water (W). The solvent used is 90 kg/hr of pure furfural (F). The relevant equilibrium ternary phase diagram is given below. Determine the number of equilibrium transfer units required to achieve an overall raffinate composition of 0.10, that is, xRN = 0.10.

5.9

Liquid–Liquid Extraction

385

Solution Draw the schematic for this multistage extraction process along with all the given information.

Obtain the composition of the mixture resulting from the feed and the solvent by mass balances and then locate the mixture point, M, on the phase diagram as shown. Note that point M will be on the line joining the feed and the solvent. Overall mass balance: F þ S = M ) M = 165

kg kg þ 90 = 255 kg=hr hr hr

Component balance for solute, C: FxF,C þ SyS,C = MxM,C kg kg kg ð0:65Þ þ 90 ð0Þ = 255 ðxM,C Þ hr hr hr ) xM,C = 0:42

)

165

Since the composition of the raffinate exiting the system isxRN = 0.10, locate RN on the equilibrium curve as shown. Join RNM and extend it to the equilibrium curve to locate E1 on the phase diagram. At E1, the composition of the solute is, y1 = 0.48. Draw the tie line from E1 to the equilibrium curve and the intersection point marks the raffinate composition in equilibrium with y1. Note that the tie line is very short since point E1 is very close to the plait point. Therefore, the equilibrium concentration is xF = 0:50. Since yS = 0, the composition of the raffinate in equilibrium with yS is xRN = 0.

386

5

Mass Transfer

Substitute the known values into Eq. 5.57 to determine the number of transfer units required. NTU =

xF - xRN = ðxF - xF Þ - ðxRN - xRN Þ ln

5.9.3.2

xF - x F xRN - x RN

0:65 - 0:10 ð0:65 - 0:50Þ - ð0:10 - 0Þ - 0:50 lnð0:65 0:10 - 0 Þ

= 4:46

Finding the Number of Theoretical Stages Required Using Extraction Factor and Distribution Coefficient

This method can be used for dilute solutions, where the flow rate of the extract and raffinate solutions can be considered to be constant throughout the system [1, 5]. Consider a single-stage system shown in Fig. 5.19. Component balance for the solute results in the following equation. The flow rates can be assumed to be constant at F and S.

Fig. 5.19 Single-stage extraction for dilute solutions

5.9

Liquid–Liquid Extraction

387

Fig. 5.20 Multistage extraction system handling dilute solutions

S x K = 0 -1 x F D

ð5:58Þ

In Eq. 5.58, KD = yx. KD is called the distributioncoefficient. Equation 5.58 can be written in the following format. S K =E F D

ð5:58aÞ

In Eq. 5.58a, E is called as the extraction factor. From Eq. 5.58 and using the definition of the extraction factor, E=

x x0 -1 ) E þ 1= 0 ) x x

x 1 = x0 1 þ E x -x x =1=1 Fraction solute recovered after extraction = 0 x0 x0 1 E = 1þE 1þE Fraction solute remaining after extraction =

ð5:59Þ

ð5:60Þ

Now consider a multistage extraction system involving dilute solutions as shown in Fig. 5.20. For the multistage counterflow system with dilute solutions shown in Fig. 5.20, the number of ideal stages required can be calculated using Eq. 5.61.

N=

ln

xF - xF xN - xN

ln E

ð5:61Þ

In Eq. 5.61, xF is the raffinate concentration in equilibrium with the extract concentration, y1, and xN is the raffinate concentration in equilibrium with solvent

388

5 Mass Transfer

(extract) concentration, yS. Using the distribution coefficient, KD, and yS = 0 for a pure solvent, the following equations can be written. y y ) xF = 1 x KD y xN = S = 0 KD

KD =

ð5:62Þ ð5:63Þ

Example 5.17 The distribution coefficient for a multistage extraction system involving dilute solutions is 20. The ratio of the solvent used per unit mass of feed solution is 0.12. Determine: A. the fraction of solute recovered in a single ideal stage. B. the number of ideal stages required to recover 95% recovery of the original solute. Solution A. Calculate the extraction factor, E, using Eq. 5.58a. E=

S K = ð0:12Þð20Þ = 2:4 F D

Calculate the fraction of the solute recovered after a single ideal stage of extraction using Eq. 5.60. Fraction solute recovered after single stage of extraction =

E 2:4 = = 0:7059 1 þ E 1 þ 2:4

B. Draw the schematic diagram for multistage extraction.

With the assumption of a feed solution flow rate of 1 lbm/sec and a feed composition of 60% solute )F = 1 lbm/ sec and xF = 0.60 )

S lbm = 0:12 lbm= sec = 0:12 ) S = 0:12F = ð0:12Þ 1 F sec

5.10

Leaching

389

After 95% recovery solute, only 5% of the solute will be remaining in the raffinate coming out of the Nth stage of extraction. Therefore, xN = 0:05 ) xF xN = 0:05xF = 0:05 × 0:60 = 0:03 Overall mass balance around the red-dashed boundary results in the following equation.

xF is the raffinate concentration in equilibrium with y1, the extract concentration leaving the system. xF can be determined by using the distribution coefficient and Eq. 5.62. xF =

y1 4:75 = = 0:2375 KD 20

From Eq. 5.63, xN = 0. Calculate the number of ideal stages required using Eq. 5.61.

N=

5.10

ln

xF - xF xN - xN

ln E

=

ln

0:60 - 0:2375 0:03

lnð2:4Þ

= 2:85

Leaching

Leaching, also known as solid–liquid extraction, is a solid–liquid mass transfer process. A solvent is used in extracting a desirable component, called the solute, from a solid substrate. Typically, the leaching process consists of the following steps: • Transport of the solvent molecules from the bulk phase to the solid surface • Diffusion of the solvent into the solid structure

390

5

Mass Transfer

• Solute molecules dissolve in the solvent • The dissolved solute is transported from the solid structure to the bulk solvent Some of the important industrial applications of leaching are as follows: • Extracting metals from mined raw materials using acid solvents • Extracting oil from “oil meals,” which are solid substances such as soyabean, sunflower seeds, using organic solvents • Extracting sugar from beets using hot water as solvent • Leaching of roasted coffee beans with hot water to produce concentrated coffee extract, later crystallized to produce instant coffee • Leaching of desirable pharmaceutical components from naturally occurring substances

5.10.1

Batch Leaching Process

Batch leaching process takes place in a constant volume vessel, where the solid substrate to be leached is charged along with the solvent. The equipment ensures intimate contact between the solid substrate and the solvent resulting in the solute diffusing into the solvent. Batch leaching mechanism can be modeled using the film theory for mass transfer process. Accordingly, the resistance to mass transfer is assumed to be in a thin liquid film surrounding the solid particle to be leached. In this film, the mass concentration (kg/m3) of the solute decreases from a value equal to CS at the solid surface to a value of C at the boundary of the film, which is also the boundary of the bulk liquid solvent phase. The use of film theory enables the modeling of the mass transfer using a film mass transfer coefficient, kL, which typically has units of m/s or ft/sec. The film mass transfer coefficient is determined experimentally from pilot studies. The rate of mass transfer of the solute from the solid to the solvent is the product of the mass transfer coefficient multiplied by the interfacial area for mass transfer and the concentration difference driving force. Therefore, mass balance results in the following set of equations. Rate of decrease of mass of solute in the solid phase = rate of increase of mass of solute in the solvent⟹ V

dC = k L aðCS- C Þ dt

ð5:64Þ

Equation 5.64 is the differential equation applicable to a batch leaching process. In Eq. 5.64, V: volume of the leach mixture, that is, the volume of the charge consisting of the solid to be leached and the solvent (m3). CS: mass concentration of the solute at the solid surface (kg/m3). C: mass concentration of the solute in the solvent at any given time (kg/m3).

5.10

Leaching

391

kL: mass transfer coefficient (m/s) a: interfacial area available for mass transfer (m2) Integrate Eq. 5.64 after separating the variables with the following boundary conditions: when t = 0, C = 0 when t = t f , C = C f k a dC = L dt ) CS - C V Cf 0

dC k a = L CS - C V C

- ln ðCS - CÞj0 f = ln

tf

dt )

0

kL a t ðtÞj0f ) V

CS - Cf k a = - L tf ) CS V kL a Cf = e - ð V Þtf ) 1CS kL a Cf = 1 - e - ð V Þt f CS

ð5:65Þ

Equation 5.65 can be used in solving problems related to batch leaching process. Example 5.18 Pilot studies for leaching a solute from solid pellets indicate the following result for the product of the mass transfer coefficient and the interfacial area: k L a = 0:0264 m3 =s 200 kg of a solid substrate containing 22% of desirable water soluble solute is in intimate contact with 10,000 liters of water with good mixing of solid pellets and water. Determine the time required to remove 50% of the solute from the solid pellets and the concentration of the leach solution at that point. Solution A. After leaching 50% of the original solute, the final concentration of the solute in the leach solution is Cf =

0:50ð0:22 × 200 kgÞ = 2:2 kg=m3 1 m3 10000 L × 1000 L

392

5

Mass Transfer

B. Calculate the time required to achieve the preceding final concentration using Eq. 5.65. kL a kL a Cf Cf = 1 - e - ð V Þt f ) e - ð V Þt f = 1 ) CS CS

-

Cf kL a t = ln 1 CS V f Cf CS kL a V

ln 1 ) tf =

5.10.2

= -

ln ð1 - 0:5Þ m3 0:0264 s 1 m3 10000 L × 1000 L

= 262:55 s ð4:38 min Þ

Single-Stage Continuous Counterflow Leaching Process

Consider the single-stage counterflow leaching process shown in Fig. 5.21. The following nomenclature is applicable to Fig. 5.21. Species A is the solvent, species B is the insoluble/inert solid, and species C is the solute. x: mass fraction of solute in the overflow solution = y: mass fraction of solute in the underflow solution N: ratio of inert solids to the solution =

C AþC overflow C = AþC underflow

B A þC

L0, L1, V0, V1: mass flow rates (kg/hr, lbm/hr) Single-stage leaching problems can be solved by mass balances. It is important to note that the amount of B, the inert, insoluble solid remains unchanged during leaching operations and therefore, it can be used as a key component in mass balances. Fig. 5.21 Continuous single-stage counterflow leaching and nomenclature

5.10

Leaching

393

Example 5.19 A single-stage leaching process handles 200 kg/hr of sunflower oil seeds containing 38% oil. The solvent used is 800 L/hr of pure hexane (SG = 0.65). The underflow solution contains 1.5 kg insoluble solid/kg solution. Determine: A. the flow rates and compositions of the overflow and underflow solutions. B. the percent original solute leached by the solvent. Solution Draw the schematic diagram for the process as shown.

A. Since the seeds contain 38% of the solute (oil), the amount of insoluble solids in the feed is 62%. Therefore, kg × 0:62 = 124 kg=hr and hr kg × 0:38 = 76 kg=hr = L0 A = 200 hr

B = 200

(since L0 contains only the oil solute, C, and no solvent, A.) )y0 = 1.0 N0 =

B AþC

feed

=

124 kghrB 76 kg hrsoln

= 1:631 kg B=kg soln

The underflow solution contains 1.5 kg insoluble solid/kg solution ) N 1 = 1:5 kg B=kg soln B= L1 =

L1 N 1 ) B 124 kg insoluble solid = = 82:67ð ≃ 83Þ kg solnðA þ CÞ kg insoluble solid N1 1:5 kg soln ρhexane = SGhexane × ρwater = ð0:65Þ 1

kg = 0:65 kg=L L

Mass flow rate of solvent feed = ðvol:flow rateÞðdensityÞ )

V 0 = 800

L hr

0:65

kg = 520 kg=hr L

394

5

Mass Transfer

Overall mass balance results in the following set of equations. V 0 þ L0 = V 1 þ L1 ) V 1 = V 0 þ L0 - L1 = 520

kg kg kg þ 76 - 83 = 513 kg=hr hr hr hr

Total solution balance: V 0 þ L0 = V 1 þ L1 = M ) kg kg kg kg M = 520 þ 76 = 513 þ 83 = 596 kg=hr hr hr hr hr Component balance for the solute: V 0 x0 þ L0 y0 = MxM ) V x þ L0 y0 xM = 0 0 = M

520

kg kg ð0Þ þ 76 ð 1Þ hr hr = 0:127 kg 596 hr

The solute will distribute itself in the same proportion in the overflow and underflow solutions as its mole fraction in the mixture. Therefore, x1 = y1 = xM = 0:127 B. Calculate the solute in the leached overflow solution.

C V 1 = V 1 x1 = 513

kg soln hr

0:127

kg solute = 65:15 kg solute kg soln

Therefore, the percent original solution leached is %Leached =

65:15 kg solute CV 1 hr 100 = 100 ≃ 86% C L0 76 kg solute hr

5.10.3 Multistage Continuous Counterflow Leaching A schematic diagram for multistage counterflow leaching is shown in Fig. 5.22. The number of stages required can be calculated using Eq. 5.66.

5.10

Leaching

395

Fig. 5.22 Multistage counterflow leaching

xNþ1 - xNþ1 x1 - x1 N =1 þ - x1 x log Nþ1 xNþ1 - x1 log

ð5:66Þ

In Eq. 5.66, xNþ1 is the overflow concentration in equilibrium with the underflow concentration, yN, and (x1 ) is overflow concentration in equilibrium with the underflow concentration, y0.

5.10.4

Determining Number of Stages Required Using the R-Factor

Under the common assumption of constant underflow from each stage in a multistage counterflow leaching process, the R-factor is defined as the ratio of flow in overflow to the flow in the underflow. The R-factor is mathematically represented by Eq. 5.67. R=

amount of soln:=solute=solvent in overflow amount of soln:=solute=solvent in underflow

ð5:67Þ

If S0 is the amount of the original solute in the feed, and SN is the amount of the final solute leaving the Nth stage, then fraction of original solute in the discharge is f=

SN = fraction of original solute leaving the system S0

ð5:68Þ

If N is the number of stages required, then, N, f and R are related by the following equation. R-1 =f RN - 1

ð5:69Þ

Equation 5.69 can be solved for N to obtain the following equation for the number of stages required.

396

5

N=

log 1 þ

Mass Transfer

R-1 f

log R

ð5:70Þ

Example 5.20 3.25 kg/s of a mineral substrate containing 60% inert/insoluble solids and the rest being useful metal is leached using 1.3 kg/s of acid. The operation is conducted in a counterflow leaching process with constant underflow. The residue from each stage contains 0.10 kg acid/kg insoluble solid. Determine the number of stages required so that the solids in the underflow from the final stage will contain 4% metal using A. the R factor method. B. the equilibrium method assuming a straight line equilibrium relationship, y = x. Solution A. Determine the original solute, the final solute, and hence the fraction original solute in the discharge from the given information.

L0 = S0 = 3:25 SN = 3:25

kg substrate s

kg substrate s

0:40 kg metal = 1:30 kg metal=s kg substrate 0:04 kg metal = 0:13 kg metal=s kg substrate

Calculate the fraction of the original solute leaving the system, f, using Eq. 5.68. f=

0:13 kg solute SN s = = 0:10 S0 1:30 kg solute s

Calculate the constant mass flow rate of the insoluble/inerts in the underflow solution. B = 3:25

kg substrate s

0:60 kg inerts = 1:95 kg inerts=s kg substrate

Determine the flow rates of the overflow and underflow solutions with the help of a schematic diagram as shown.

5.10

Leaching

397

With constant underflow assumption, the mass flow rate of acid from stage N is m_ acid,N = 0:10

kg acid kg inerts

1:95

kg inerts = 0:195 kg acid=s s

The solute from stage N is 0.13 kg solute/s. The underflow from stage N, LN, is the sum of the acid (solvent) and solute flow rates. Therefore, LN = 0:13

kg solvent ðacidÞ kg solute = 0:325 kg soln:=s þ 0:195 s s

Calculate the overflow from stage 1 using overall mass balance for the solution around the red envelope. V Nþ1 þ L0 = V 1 þ LN ) V 1 = V Nþ1 þ L0 - LN = 1:30

kg kg kg þ 1:30 - 0:325 = 2:275 kg=s s s s

Calculate the ratio, R, from Eq. 5.67. R=

kg soln: V 1 2:275 s = =7 LN 0:325 kg soln: s

Determine the number of stages required using Eq. 5.70. N=

-1 log ½1 þ ðR -f 1Þ log ½1 þ ð70:1 Þ = 2:11 = log 7 log R

Hence, N > 2, that is three (3) theoretical stages are required. B. Using the equilibrium method, the number of stages required can be determined by using Eq. 5.66 reproduced here for reference.  xNþ1 - xNþ1  x1 - x1 N =1 þ xNþ1 - x1 log  xNþ1 - x1

log

398

5

Mass Transfer

In the preceding equation, xNþ1 is the overflow concentration in equilibrium with the underflow concentration, yN, and (x1 ) is overflow concentration in equilibrium with the underflow concentration, y0. Referring to the schematic diagram shown in the solution to part A, xN + 1 = 0 since pure solvent (acid) is used. Determine yN, the mass fraction of the solute in the underflow solution from stage N, LN using the results from the solution to part A, that is, LN = 0:325 kg soln=s, with 0:13 kg solute=s yN =

C LN 0:13 kg solute=s = = 0:40 0:325 kg solution=s ðA þ CÞLN

Determine xN þ1 from the equilibrium relationship, y = x ) xN þ1 = yN = 0:40 Determine x1, the mass fraction of the solute in the overflow solution from stage 1 using results from the solution to part A, that is, V1 = 1.56 kg solution/s. The feed, L0, has 1.30 kg solute/s and the underflow from stage N, LN, has 0.13 kg solute/s. Therefore, from mass balance for the solute, the solute in the overflow solution, V1 is CV1 = 1:30

kg soln kg soln - 0:13 = 1:17 kg soln=s s s

As determined in the solution to part A, the total flow rate of the overflow solution from stage 1, that is, V1 is 2.275 kg solution / s. Therefore, x1 =

CV 1 1:17 kg solute=s = = 0:51 2:275 kg solution=s ðA þ CÞV 1

Determine x1 from the equilibrium relationship, y = x ) x1 = y0 = 1:0 Substitute all the known values into the equation for the required number of stages.

N =1 þ

log log

xNþ1 - xNþ1 x1 - x1 xNþ1 - x1 xNþ1 - x1

=1 þ

0 - 0:40 log ½0:51 - 1:0 = 2:25 0 - 0:51 log ½0:40 - 1:0

Hence, N > 2, that is three (3) theoretical stages are required. The number of required stages (2.11 vs. 2.25) obtained by the two methods are in good agreement with each other.

5.11

Adsorption Process

399

5.11 Adsorption Process Adsorption is either a solid–gas unit operation or a solid–liquid unit operation where the undesirable component in a gas or liquid mixture is preferentially adsorbed by a solid solvent [1, 5]. The solid used for adsorption is known as the adsorbent or solvent and the component being adsorbed is known as the adsorbate or solute. The following examples illustrate the applications of adsorption operations: • Use of silica gel to remove moisture primarily from gases to facilitate operations requiring dry gas • Use of activated carbon to adsorb components causing bad taste and/or bad odor, especially from water • Use of specific adsorbents to remove components that cause competing reactions in reactive systems or components that cause catalyst poisoning in catalytic reactions • Use of activated carbon to recover valuable solvent vapors Typically, adsorbents must have a porous structure offering a large surface area per unit mass of adsorbent.

5.11.1

Adsorption Equilibrium

Consider the adsorption of a component (adsorbate) from a gas mixture. Obviously, the partial pressure, P, of the adsorbate quantifies the relative amount of adsorbate in the mixture, and it is reasonable to expect the mass of the component adsorbed to be proportional to the partial pressure, P, of the adsorbate. The amount of a solute adsorbed is usually quantified in terms of a ratio of mass of solute adsorbed per unit mass of adsorbent. Mathematically, the preceding ratio can be represented as Rm =

mass of solute adsorbed mS = mA mass of adsorbent

ð5:71Þ

The adsorption equilibrium relationship for adsorption of gaseous components from a gas mixture is commonly represented by the Freundlich equation, Eq. 5.72. Rm =

1 mS = KPn mA

ð5:72Þ

In Eq. 5.72, P is the equilibrium partial pressure of the component being adsorbed, and K, n, are empirical constants to be determined from experiments [2]. Taking the log of both sides of Eq. 5.72 results in the following equation. log

1 mS log P = log K þ n mA

ð5:73Þ

400

5

Mass Transfer

Thus, a log-log plot of mass solute adsorbed per unit mass adsorbent vs. the partial pressure of the component being adsorbed will be a straight line with slope 1n, and y-intercept, logK, the parameters in the Freundlich equation. Note: The Freundlich equation can be used for adsorption of components from liquid mixtures also. In this case, the amount of the component to be adsorbed in the mixture is represented in terms of the concentration of the component, C (g/L, kg/m3, lbm/ft3) and the plot will be mass of solute adsorbed per unit mass of absorbent vs. concentration of the component being adsorbed [1, 2, 5]. Example 5.21 An organic compound is to be removed from effluent water using activated carbon as the adsorbent. The following equilibrium data is available from experimental studies (mS is the mass of solute adsorbed and mA is the mass of the adsorbent used) : C (mg/L) mS/mA(mg/mg)

1.5 0.008

3.0 0.021

6.0 0.035

9.0 0.048

12 0.065

15 0.081

A. Determine the Freundlich parameters and hence the Freundlich equation. B. If 1800 L/min of effluent with 10 mg/ L of organic compound is fed to the adsorption unit, determine the mass feed rate of activated carbon required to remove 80% of the organic compound from the effluent. Solution Obtain the log of the given data and include it in the data table A C (mg/L) mS/mA (mg/mg) log C log (mS/mA)

1.5 0.008 0.1761 -2.097

3.0 0.021 0.4771 -1.678

6.0 0.035 0.7781 -1.456

9.0 0.048 0.9542 -1.319

12 0.065 1.079 -1.187

15 0.081 1.176 -1.091

Plot log(mS/mA) vs. log C and obtain the slope of the resulting straight line.

5.11

Adsorption Process

Slope =

401

- 1:26 - ð - 1:80Þ Δy 1 =1= ) n=1 = 1 - 0:46 Δx n

Obtain the value of K by taking a point on the graph and substituting the known values into Eq. 5.73 with P replaced by C. 1 mS = log K þ log C ) mA n

log

log K = log

mS 1 1 log C = - 1:32 ð0:95Þ = - 2:27 ) mA n 1

K = 10 - 2:27 = 0:0054 Using Eq. 5.72 as a template for Freundlich equation and the parameters just obtained, the Freundlich equation applicable in this situation is 1 mS = KC n mA

n = 1, K = 0:0054

→

mS = 0:0054C mA

B. Determine the mass rate of the solute (80% of incoming) to be adsorbed from the given data. _ = 1800 m_ S = VC

L min

10

mg ð0:8Þ = 14400 mg= min L

From the Freundlich equation, m_ S mg = 0:0054C = 0:0054 × 10 = 0:054 ) m_ A L mg 14400 m_ S min = = 2:667 × 105 mg= min ð266:7 kg= min Þ m_ A = 0:054 0:054

5.11.2

Continuous Multistage Counterflow Adsorption

Consider a continuous adsorption process occurring in an adsorption unit consisting of multiple equilibrium or theoretical stages. The entity containing the component to be adsorbed enters the unit from the bottom of the unit and the adsorbent (usually pure) enters from the top of the tower [5]. The schematic diagram and nomenclature are similar to those used in counterflow gas absorption. The schematic diagram is shown in Fig. 5.23 followed by the applicable nomenclature.

402

5

Mass Transfer

The following nomenclature is applicable to Fig. 5.23. G1: mass flow rate of the solution entering with more solute (kg/min) G2: mass flow rate of the solution leaving with less solute (kg/min) GS: mass flow rate of the solute-free gas/liquid entering and leaving (kg/min) L2: mass flow rate of the adsorbent entering with less or no solute (kg/min) L1: mass flow rate of the adsorbent leaving with more solute (kg/min) LS: mass flow rate of the solute-free adsorbent entering and leaving (kg/min) xA1: mass fraction of the solute in the adsorbent leaving the tower (mass solute/mass adsorbent + solute) xA2: mass fraction of the solute in the adsorbent entering the tower (mass solute/mass adsorbent + solute)xA2 = 0, for pure adsorbent XA1: mass ratio in the adsorbent leaving the tower (mass solute/mass adsorbent) XA2: mass ratio in the adsorbent entering the tower, XA2 = 0for pure adsorbent, (mass solute/mass adsorbent) yA1: mass fraction of the solute in the solution entering the tower (moles solute/mass solution) yA2: mass fraction of the solute in the solution leaving the tower (moles solute/mass solution) YA1: mass ratio in the solution entering the tower (mass solute/mass solute ‐ free gas or liquid) YA2: mass ratio in the gas leaving the tower (mass solute/mass solute ‐ free gas or liquid) The relationship between the mass fractions and mass ratios are

Fig. 5.23 Continuous counterflow adsorption

5.11

Adsorption Process

403

xA 1 - xA yA YA = 1 - yA

XA =

ð5:74Þ ð5:75Þ

The material balance equations for the component being adsorbed, A, are all based on the following mass balance equation. mass of A entering the tower = mass of A leaving the tower

ð5:76Þ

G1 yA1 þ L2 xA2 = G2 yA2 þ L1 xA1

ð5:77Þ

GS Y A1 þ LS X A2 = GS Y A2 þ LS X A1 )

ð5:78Þ

GS ðY A1- Y A2 Þ = LS ðX A1- X A2 Þ )

ð5:78Þ

ðY - Y A2 Þ LS = A1 GS ðX A1 - X A2 Þ

ð5:79Þ

Equation 5.79 will provide the slope of operating line for the adsorption unit. Once the equilibrium curve is known, a stage-wise construction method (similar to the method used in gas adsorption) can be used in obtaining the number of theoretical stages as illustrated in the following example. Example 5.22 1100 lbm/hr of air with 0.5% moisture is to be partially dried to a final moisture content of 0.001 lbm H2O/lbm d a in a continuous adsorption column. The desiccant used is pure silica gel with a feed rate of 40 lbm/hr in a direction opposite to the air flow direction. The equilibrium relationship is, Y = - 0.046X2 + 0.04X, where, X = lbm H2O adsorbed/lbm dry silica gel, and Y = lbm H2O/lbm dry air. Determine: A. the moisture content of the silica gel leaving the dryer. B. the number of theoretical stages required. Solution Represent all the known quantities on a schematic diagram for the process.

404

5

Mass Transfer

Calculate the mass flow of dry air and the mass ratio of moisture to dry air at the tower entrance (reference point 1). GS = 1100

lbm moist air hr

yA1 Y A1 = = 1 - yA1

0:995 lbm dry air = 1095 lbm dry air=hr lbm moist air

lbm H2 O lbm moist air = 0:005 lbm H2 O=lbm dry air lbm dry air ð1 - 0:005Þ lbm moist air 0:005

A. Calculate the mass ratio of moisture to dry silica gel in the silica gel leaving the tower using the material balance equation, Eq. 5.78. GS ðY A1 - Y A2 Þ = LS ðX A1 - X A2 Þ ) GS X A1 = ðY A1 - Y A2 Þ þ X A2 LS lbm dry air 1095 lbm H2 O lbm H2 O hr þ0 = × 0:005 - 0:001 lbm dry silica gel lbm dry air lbm dry air 40 hr = 0:1095 lbm H2 O=lbm dry silica gel

5.12

Crystallization

405

B. For different values of X, calculate Y using the given equilibrium relationship and draw the equilibrium curve based on the following table. X= Y=

lbm H2 O lbm dry gel lbm H2 O lbm dry air

0

0.04

0.08

0.12

0.16

0.20

0

0.0015

0.0029

0.0041

0.0052

0.0062

Draw the operating line by connecting ðX A2= 0, Y A2 = 0:001Þ with ðX A1= 0:1095, Y A1 = 0:005Þ Step off the stages as shown to obtain four (4) required theoretical stages.

5.12 Crystallization Crystallization is a unit operation where a solution is cooled to create a supersaturated state causing the excess solute to crystallize out of the solution [2, 3]. As an example, the solubility of sugar in water increases with temperature. Therefore, when a homogeneous sugar solution is cooled to a significant degree it becomes supersaturated and the amount of sugar in excess of the solubility at the lower temperature comes out of the solution as crystalline solids. The crystals can be separated from the solution by another unit operation, namely, filtration. Crystallization unit operation is widely used in the chemical, food, and pharmaceutical industries. Examples of applications of crystallization process to produce the listed products in each industry are listed here [3].

406

5

Mass Transfer

• Sodium chloride, potassium chromate/dichromate, ammonium sulfate, melamine, and magnesium sulfate (chemical industry) • Benzoic acid, dextrose, and lactose (food industry) • Antibiotics, vitamins, and various drugs (pharmaceutical industry)

5.12.1

Crystallization Process Calculations

The calculations required for a typical crystallization process involve the use of data such as water of crystallization, solubilty, and heat of solution [2, 3]. The process calculations can be accomplished by simple mass and heat balances as illustrated in the following example problem. Example 5.23 5000 lbm/hr of saturated magnesium sulfate (MgSO4) solution at 130 °F is cooled to 70 °F in a jacketed crystallizer to produce magnesium heptahydrate crystals. The slurry, consisting of crystals and the associated solution, leaves the crystallizer at 70 °F. Cooling water enters the crystallizer jacket at 60 °F and leaves at 80 °F. The solubility and heat of crystallization data as well as the relevant thermodynamic and heat transfer data are as follows: Solubility of MgSO4 at 130°F = 0.481 lbm MgSO4/lbm water Solubility of MgSO4 at 70 °F = 0.355 lbm MgSO4/lbm water Heat of crystallization at 70 °F = 5726 Btu/lbmol MgSO4  7H2O Molecular weight of MgSO4 = 120 lbm/lbmol Molecular weight of MgSO4  7H2O = 246 lbm/lbmol Specific heat of feed and all solutions = 0.70 Btu/lbm-°F Overall heat transfer coefficient for the jacketed surface = 43 Btu/hr-ft2-°F Cooling surface area available per vessel = 32.5 ft2 Determine: A. B. C. D.

the rate of production of the heptahydrate crystals. the mass flow rate of cooling water required. the heat duty of the jacketed vessel. the number of crystallizer vessels required.

Solution Draw the schematic diagram for crystallization/cooling process. The subscripts used are F: feed, C: heptahydrate crystals of magnesium sulfate, S: solution part of the slurry from the crystallizer/cooler, M: MgSO4, CW: cooling water

5.12

Crystallization

407

Convert the given solubility data to solution basis, that is mass fraction MgSO4 in the solution. At 130 °F, lbm MgSO4 1 lbm water lbm water 1:481 lbm soln = 0:3248 lbm MgSO4 =lbm soln

xMF = 0:481

At 70 °F, lbm MgSO4 1 lbm water lbm water 1:355 lbm soln = 0:2620 lbm MgSO4 =lbm soln

xMS = 0:355

A. Perform mass balances around the red envelope. Overall mass balance: m_ F = m_ C þ m_ S MgSO4 balance:

m_ F = 5000 lbm=hr

→ m_ S = ð5000- m_ C Þ

lbm soln hr

408

5

m_ F xMF = ðm_ C lbmcrystalsÞ 5000

lbmsoln hr

Mass Transfer

m_ S =5000- m_ C 120lbmMgSO4 → þ m_ S xMS 246lbmcrystals

120lbmMgSO4 0:3248lbmMgSO4 = ðm_ C lbmcrystalsÞ 246lbmcrystals lbmsoln

lbmMgSO4 120 lbmsoln 0:2620lbmMgSO4 )1624 lbmsoln hr 246 hr lbmMgSO4 lbmMgSO4 lbmMgSO4 þ1310 -0:1278m_ C =0:4878m_ C hr hr hr m_ C =872lbmheptahydratecrystals=hr,and lbm lbm lbmcrystals m_ S =5000 - m_ C =5000 -872 =4128lbmsoln:=hr hr hr hr þ 5000- m_ C

B. Determine the enthalpies of cooling water from steam tables or online steam property resources. (Source: https://www.spiraxsarco.com/resources-anddesign-tools/steam-tables/sub-saturated-water-region#article-top) hCW,in = hf ð60 ° FÞ = 27:9 Btu=lbm hCW,out = hf ð80 ° FÞ = 47:9 Btu=lbm Convert the given heat of crystallization to mass basis. hcryn: =

Btu 5726 lbmol hcryn: = 23:28 Btu=lbm = lbm M crystals 246 lbmol

Overall energy balance results in the following set of equations. Rate of heat input þ rate of heat released due to crystallization = rate of heat output

5.12

Crystallization

409

m_ F cP T F þ m_ CW hCW,in þ m_ C hcryn: = m_ S cP T S þ m_ CW hCW,out ) cP ðm_ F T F - m_ S T S Þ þ m_ C hcryn: m_ CW = hCW,out - hCW,in Btu lbm lbm 5000 0:70 × 130 ° F - 4128 × 70 ° F lbm‐ ° F hr hr Btu lbm = Btu Btu 47:9 - 27:9 lbm water lbm water = 13, 651 lbm water=hr þ 872

lbm hr

23:28

C. For the crystallizer/cooler jacketed vessel, calculate the log mean temperature difference by using the temperature schematic shown below and Eq. 4.36. 1 130 ° F

2 ! 70 ° F

Solution CW

60 ° F ! 80 ° F ΔT2 = 10 ° F ΔT1 = 70 ° F ΔT LMTD =

ΔT 1 - ΔT 2 ln

ΔT 1 ΔT 2

=

70 ° F - 10 ° F = 30:83 ° F °F ln 70 10 ° F

Note: The average temperature of the solution is (130 °F + 70 °F)/2 = 100 °F and the average temperature of cooling water is (60 °F + 80 °F)/2 = 70 °F. Therefore, the average temperature difference driving force of 100 °F – 70 ° F = 30 °F can also be used in the calculations. Calculate the heat duty available from each vessel from the given data. From Sect. 4.4.4 in Chap. 4, the heat exchanger design equation (Eq. 4.44) is qvessel = UAs ΔT LMTD

Btu 32:5 ft2 ð30:83 ° FÞ hr‐ft2 ‐ ° F = 43, 085 Btu=hr = 43

D. Calculate the total heat duty of the crystallization/cooling process under the conditions specified. This will be equal to the rate of heat absorbed by the cooling water.

410

5

Mass Transfer

= ðm_ CW ÞðhCW,out - hCW,in Þ lbm Btu Btu = 13651 47:9 - 27:9 hr lbm lbm = 273, 020 Btu=hr

qtotal

Calculate the number of jacketed vessels required. Number of vessels required =

273020 Btu qtotal hr = = 6:34 qvessel 43085 Btu hr

Therefore, seven (7) identical crystallizer/cooler vessels are required to handle the given situation. The feed and the cooling water can be evenly distributed between the seven vessels.

5.13

Evaporation

Evaporation is a unit operation used in producing a concentrated solution from a dilute feed by evaporating water. Steam is typically used as the heat source required to evaporate the water and the concentrated solution typically leaves the evaporator at its boiling point [2, 3]. Evaporators are usually vertical vessels, and the steam circulation can take place in a surrounding jacket or a dedicated steam chest, which is essentially a shell and tube heat exchanger where the liquid to be concentrated circulates in the tubes and steam condenses on the shell side. Evaporators are extensively used in the food industry to make concentrated sugar solutions, and in making concentrated coffee extract, which is subsequently freezedried to produce instant coffee crystals [2, 3]. Evaporators can be a single unit (Single-Effect Evaporator), or they can consist of multiple similar units (MultipleEffect Evaporators). The Evaporator Economy, E, is a performance measure of the evaporator and it is defined as the mass of water evaporated per unit mass of steam consumed. E=

5.13.1

mass water evaporated m_ w,evap = m_ steam mass steam consumed

ð5:80Þ

Single-Effect Evaporator

The schematic diagram of a single-effect evaporator is shown in Fig. 5.24 Evaporator calculations can be accomplished by using simple mass and energy balances and equations related to heat exchangers.

5.13

Evaporation

411

Fig. 5.24 Schematic diagram for a single-effect evaporator

Example 5.24 It is desired to produce 3000 kg/hr frozen orange concentrate from a solution of orange juice, which is to be concentrated from 20% pulp to 50% pulp. The heating medium is condensing steam at 250 kPa. The evaporator pressure is 30 kPa (abs). The overall heat transfer coefficient is 2270 W/m2-°C. The feed enters the evaporator at 20 °C and the specific heat of the feed solution is 3.56 kJ/kg-°C. The enthalpy of vaporization of the solution is identical to that of water at the given temperature or pressure. Determine: A. B. C. D. E. F.

the required feed rate. the rate at which water is removed. the heat duty of the steam chest. the required supply rate of steam. the heat transfer surface area required in the steam chest. the economy of the evaporator.

Solution Draw the schematic diagram for the process.

412

5

Mass Transfer

A. Component balance for the pulp around the red envelope gives the following result. FxF = PxP ) F =

kg PxP 3000 hr × 0:50 = = 7500 kg=hr xF 0:20

B. Calculate the rate of evaporation of water using overall mass balance around the red envelope. F=P þ V ) V = F - P = 7500

kg kg = 4500 kg=hr - 3000 hr hr

C. Determine the saturation temperature, TSat, and the enthalpy of vaporization of steam at 30 kPa from steam tables or online steam property resources (Source: https://www.spiraxsarco.com/resources-and-design-tools/steam-tables/dry-satu rated-steam-line#article-top). At 30 kPa, T Sat = 69 ° C, and hfg = 2335 kJ=kg Note that the saturation temperature at 30 kPa is also the boiling point of the solution and it represents the temperature of the product solution from the evaporator. If q represents the rate of heat input from the steam (also the heat duty of the steam chest), then energy balance for the entire system including steam results in the following set of equations. Rate of energy in = rate of energy out ) FhF þ q = VhV þ PhP ) q = VhV þ PhP - FhF

P=F-V

! q = VhV þ ðF - V ÞhP - FhF = V ðhV - hP Þ þ F ðhP - hF Þ q = Vhfg þ FcP ðT P - T F Þ = 4500

kg hr

2335

kJ kg þ 7500 kg hr

3:56

kJ ð69 ° C - 20 ° CÞ kg  ° C

= 1:1816 × 107 kJ=hr D. Steam is supplied at 250 kPa absolute pressure. Determine the saturation temperature, TSteam, and the enthalpy of condensation of steam at 250 kPa from steam tables or online steam property resources. (Source: https://www. spiraxsarco.com/resources-and-design-tools/steam-tables/dry-saturated-steamline#article-top).

5.13

Evaporation

413

At 250 kPa, T Steam = 127 ° C, and hcondn = 2181 kJ=kg Heat supplied by steam, q = m_ S hcondn,S ) m_ S =

q hcondn,S

=

1:1816 × 107 kJ hr = 5418 kg=hr kJ 2181 kg

E. Calculate the heat transfer surface area required in the steam chest using the heat exchanger design equation, Eq. 4.44. Note that the temperature difference is the difference between the temperature of steam and the average temperature of the solution. The average temperature of the solution is, Tavg,soln = (TF + TP) / 2 = (20 C + 69 C) / 2 = 44.5 C. q = UAS ΔT ) J 103 kJ kJ 1:1816 × 10 × hr 3600 s q hr = 17:53 m2 AS = = ∘ ∘ W UΔT 2270 2 ð127 C - 44:5 CÞ m  °C 7

Note: W  J/s F. Calculate the evaporator economy, E, using Eq. 5.80. E=

5.13.2

4500 kg mass water evaporated m_ V hr = = 0:8306 = mass steam consumed m_ steam 5418 kg hr

Multiple-Effect Evaporator

A multiple-effect evaporator uses multiple units and the vapor from the first unit serves as the input steam for the second unit and the product from the first unit becomes the feed for the second unit and so on. Hence, there is much greater amount of evaporation for the same amount of steam input [2, 3]. This results in significantly higher evaporator economy. A schematic diagram for a multiple-effect (triple-effect) evaporator system is shown in Fig. 5.25. Example 5.25 A double-effect evaporator system is used in concentrating 3 kg/s of 10% by mass of sodium hydroxide solution to a final concentration of 30%. The first effect is designed to increase the concentration of sodium hydroxide solution to 20% by mass. The feed enters the first effect at 30 °C, and the first effect operates at a

414

5

Mass Transfer

Fig. 5.25 Schematic diagram for a triple-effect evaporator system

pressure of 70 kPa. The boiling point elevations (relative to water) in the first and second effects can be assumed to be 5 °C and 8.5 °C respectively. The overall heat transfer coefficient of 1860 W/m2.°C. Steam at 120 °C is used as the heating medium. The enthalpies can be approximated by the enthalpies from the steam tables. Determine: A. the mass flow rate of steam required in the first effect. B. the heat transfer area required in the first effect. C. the pressure required in the second effect assuming that the same evaporator as in the first effect is also used in the second effect. Compare the evaporator economies of single- and double-effect evaporation assuming a boiling point elevation of 7 °C for single-effect evaporation. Solution Draw the schematic diagram for the double-effect evaporator system as specified in the problem statement.

A. For the first effect, component balance for sodium hydroxide around the brown envelope gives the following result.

5.13

Evaporation

415

FxF = ðP1ÞxP1 ) P1 =

3:0 kgs × 0:10 FxF = = 1:5 kg=s xP1 0:20

Calculate the rate of evaporation of water from the first effect using overall mass balance around the brown envelope. F = P1 þ V1 ) V1 = F - P1 = 3:0

kg kg - 1:5 = 1:5 kg=s s s

Determine the saturation temperature, TSat, of steam at 70 kPa from steam tables or online steam property resources (Source: https://www.spiraxsarco.com/resourcesand-design-tools/steam-tables/dry-saturated-steam-line#article-top). At 70 kPa, T Sat = 90 ° C After factoring in the given boiling point elevation of 5 ° C in the first effect, the effective saturation temperature for the first effect is T Sat1 = 90 ° C þ 5 ° C = 95 ° C Note that the effective saturation temperature, TSat1 = 95 ° C is also the boiling point of the solution and it represents the temperature of the product solution from the first effect. Let q1 represent the rate of heat input from the steam in the first effect, then energy balance for the first effect, including steam, results in the following equations. FhF þ q1 = ðV1ÞhV1 þ ðP1ÞhP1 ) q1 = ðV1ÞhV1 þ ðP1ÞhP1 - FhF Determine the enthalpies from the steam tables and substitute all the known values into the preceding energy balance equation to obtain the rate of heat input from steam in the first effect. hV1 = hg at 95 ° C = 2667 kJ=kg hP1 = hf at 95 ° C = 397:6 kJ=kg hF = hf at 30 ° C = 125:3 kJ=kg

416

5

q1 = ðV1ÞhV1 þ ðP1ÞhP1 - FhF kg kJ kg = 1:5 2667 þ 1:5 s kg s = 4221 kJ=s ð4221 kWÞ

397:6

kJ kg - 3:0 kg s

Mass Transfer

125:3

kJ kg

Steam is supplied at 120 °C. Determine the enthalpy of condensation of steam at 120 °C from steam tables or online steam property resources. (Source: https://www. spiraxsarco.com/resources-and-design-tools/steam-tables/dry-saturated-steamline#article-top). At

T sat = 120 ° C, hcondn = 2202 kJ=kg

Heat supplied by steam, q1 = m_ S hcondn,S ) m_ S =

kJ s = 1:917 kg=s = kJ 2202 kg 4221

q1 hcondn,S

B. Calculate the heat transfer surface area required in the steam chest using the heat exchanger design equation, Eq. 4.44. Note that the temperature difference is the difference between the temperature of steam and the average temperature of the solution. The average temperature of the solution is, Tavg,soln = (TF + TP) / 2 = (30 C + 95 C) / 2 = 62.5 C. q1 = UAS ΔT ) q AS = 1 = UΔT

4221

J kJ × 103 kJ s

W 1860 2 ∘ ð120∘ C - 62:5∘ CÞ m  C

= 39:47 m2

C. The second effect increases the mass fraction of sodium hydroxide from 20% to 30%. The vapor from the first effect, V1 = 1.5 kg/s with TSat1 = 95 ° C, serves as the heating medium for the second effect. For the second effect, component balance for sodium hydroxide around the orange envelope gives the following result. ðP1ÞxP1 = PxP ) P =

kg ðP1ÞxP1 1:5 s × 0:20 = = 1:0 kg=s xP 0:30

5.13

Evaporation

417

Calculate the rate of evaporation of water from the second effect using overall mass balance around the orange envelope. P1 = P þ V2 ) V2 = P1 - P = 1:5

kg kg - 1:0 = 0:5 kg=s s s

The pressure in the second effect will have to be determined by the following trial and error procedure. • Assume a pressure for the second effect • Determine the saturation temperature from the steam tables, add the boiling point elevation of 8.5 °C to obtain the effective saturation temperature, which is also the boiling point of the solution in the second effect and the temperature of the product solution from the second effect • Calculate the heat duty from both the heat exchanger design equation and energy balance for the second effect • Continue until consistent values are obtained from both the design equation and energy balance Let q2, DE represent the heat duty obtained from the heat exchanger design equation and let q2, EB represent the heat duty obtained from energy balance equation. Assume a pressure of 30 kPa. Determine the saturation temperature, TSat, at 30 kPa from steam tables or online steam property resources (Source: https:// www.spiraxsarco.com/resources-and-design-tools/steam-tables/dry-saturatedsteam-line#article-top). At 30 kPa, TSat = 69.1 ° C, and after adding the boiling point elevation the effective saturation temperature for the second effect is T Sat2 = 69:1 ° C þ 8:5 ° C = 77:6 ° C Energy balance for the second effect results in the following equation. ðP1ÞhP1 þ q2,EB = ðV2ÞhV2 þ PhP ) q2,EB = ðV2ÞhV2 - ðP1ÞhP1 þ PhP Determine the enthalpies from the steam tables and substitute all the known values into the preceding energy balance equation to obtain the rate of heat input in the second effect from energy balance.

418

5

Mass Transfer

hV2 = hg at 77:6 ° C = 2638:5 kJ=kg hP1 = hf at 95 ° C = 397:6 kJ=kg hP = hf at 77:6 ° C = 324:4 kJ=kg q2,EB = ðV2ÞhV2 - ðP1ÞhP1 þ PhP kJ kg kg 2638:5 - 1:5 kg s s = 1047:2 kJ=s ð1047 kWÞ

= 0:5

397:6

kJ kg þ 1:0 kg s

324:4

kJ kg

Determine the heat duty in the second effect using the heat exchanger design equation, Eq. 4.44. Since an evaporator similar to the first effect is used, use the heat transfer area of the first effect. Note that the temperature difference is the difference between the temperature of condensing vapor from the first effect and the effective saturation temperature of the second effect. q2,DE = UAS ΔT = 1860

W 1 kW × ð39:47 m2 Þð95 ° C - 77:6 ° CÞ m2  ° C 1000 W

= 1277:4 kW Since the heat duty from the two methods are not in agreement, try P2 = 34 kPa ) ∘ T sat = 72 C þ8:5 C, BPE → T sat2 = 80:5 C, ∘

hP = hf

at 80:5∘ C

∘

at 80:5∘ C

→

= 337 kJ=kg and hV2 = hg at 80:5∘ C = 2644 kJ=kg )

Substituting the preceding and all other known values into the energy balance and heat exchanger design equations, the following results are obtained. q2,EB = 1063 kW and q2,DE = 1064 kW The results are in good agreement. Hence, the pressure required in the second effect is approximately 34 kPa, absolute. Comparison of single-effect and double-effect: Determine the equivalent steam consumption in a single-effect (SE) evaporator in increasing the mass fraction of sodium hydroxide from 10 to 30%. For a single-effect evaporator,

5.13

Evaporation

419

FxF = ðPÞxP ) P =

3:0 kgs × 0:10 FxF = = 1:0 kg=s xP 0:30

Calculate the rate of evaporation of water from a single-effect evaporator using overall mass balance. F=P þ V ) V = F - P = 3:0

kg kg - 1:0 = 2:0 kg=s s s

qSE represents the rate of heat input from the steam in the first effect, then energy balance for the first effect, including steam, results in the following equations. FhF þ qSE = VhV þ PhP ) qSE = VhV þ PhP - FhF With a boiling point elevation of 7 °C, the equivalent saturation temperature for a single-effect evaporator is TSat, SE = 90 ° C + 7 ° C = 97 ° C. Determine the enthalpies from the steam tables and substitute all the known values into the preceding energy balance equation to obtain the rate of heat input from steam in a single-effect evaporator. hV = hg at 97 ° C = 2670 kJ=kg hP = hf at 97 ° C = 406 kJ=kg hF = hf at 30 ° C = 125:3 kJ=kg qSE = VhV þ PhP - FhF kg kJ kg = 2:0 2670 þ 1:0 s kg s

406

kJ kg

kg kJ 125:3 s kg = 5370 kJ=s ð5370 kWÞ - 3:0

Steam is supplied at 120 °C. Determine the enthalpy of condensation of steam at 120 °C from steam tables or online steam property resources. (Source: https://www. spiraxsarco.com/resources-and-design-tools/steam-tables/dry-saturated-steamline#article-top). At T sat = 120 ° C, hcondn = 2202 kJ=kg

420

5

Mass Transfer

Required heat input calculated from energy balance = Heat supplied by steam qSE = m_ S,SE hcondn,S ) m_ S,SE =

qSE hcondn,S

kJ s = 2:439 kg=s = kJ 2202 kg 5370

Compare evaporator economies for single-effect (SE) and double-effect (DE) evaporators using Eq. 5.80. E SE =

2 kgs m_ w,evap,SE = = 0:8200 m_ steam,cons,SE 2:439 kgs

E DE =

2 kgs m_ w,evap,DE = = 1:043 m_ steam,cons,DE 1:917 kgs

Hence, the evaporator economy has increased by 27% by using a double-effect evaporator in place of a single-effect evaporator.

Practice Problems Practice Problem 5.1 In an absorption tower, sulfur dioxide is being absorbed by water. At a particular location in the tower, the partial pressure of sulfur dioxide in the air stream was measured to be 45 mm Hg, and concentration of sulfur dioxide in water was determined to be 0.0385 lbmol SO2/ft3soln. The tower operating conditions are 1 atm and 70 °F. The individual mass transfer coefficients are: k L = 0:985 lbmol SO2 =hr‐ft2 ‐ lbmol SO2 =ft3 soln and kG = 0:195 lbmol SO2 =hr‐ft2 ‐atm The equilibrium relationship is PAi = 0:137CAi , PAi ðatmÞ, CAi lbmol SO2 =ft3 soln :

Practice Problems

421

Determine: A. the percentage of the total resistance in the liquid film and comment on the result obtained. B. PAG - PA C. C A - C AL D. PAG - PAi, and CAi - CAL

Practice Problem 5.2 Carbon dioxide is absorbed from an air stream using a pure amine solution (molecular weight 32 kg/kmol) as a solvent. The amine solution is fed from the top of the tower (diameter 0.85 m) at a mass flow rate of 0.8 kg/s. The gas mixture containing 6 mole% carbon dioxide enters from the bottom of the tower at a molar flow rate of 0.0045 kmol/s. The desired mole fraction of carbon dioxide in the exit air stream is 1.25%. The equilibrium relationship is YA1 = 5.95XA1 where YA1 is the mole ratio of carbon dioxide to air in the gas stream and XA1 is the mole ratio of carbon dioxide to amine in the liquid stream. Determine: A. the mole fraction of carbon dioxide in the amine solution leaving the tower B. the ratio

LS GS actual to

LS GS min

Practice Problem 5.3 Given the McCabe–Thiele diagram for a binary distillation system, label all the lines/curves in the diagram with standard nomenclature and determine: A. B. C. D. E.

the minimum reflux ratio. the ratio between the actual reflux ratio and the minimum reflux ratio. the value of q and the feed condition. the liquid to vapor ratio, that is, L/V, in the rectifying section. the boil-up ratio, that is, V′/B referring to the reboiler.

422

5

Mass Transfer

Practice Problem 5.4 A wet solid with a mass of 5 kg and 10% moisture content is dried in a batch drier to final moisture content of 0.05 kg water/kg dry solid, a process which takes 4.8 hrs. The surface area of the wet solid is 1.8 m2 and the equilibrium moisture content is 0.02 kg water/kg dry solid. Assuming a constant rate of drying, determine the drying rate in kg/m2.min.

Practice Problem 5.5 The rate of water removal during the constant drying period of 0.73 hr of a wet solid is 8 lbm H2O/min-ft2. During this period, the moisture content is reduced from an initial value of 0.35 lbm H2O/lbm dry solid to critical value of 0.14 lbm H2O/lbm dry solid. The surface area of the wet solid is 2.25 ft2. The equilibrium moisture content is 0.04 lbm H2O/lbm dry solid. Determine: A. the mass of the dry solid. B. the total drying time required to achieve a final moisture content of 0.07 lbm H2O/lbm dry solid.

Practice Problems

423

Practice Problem 5.6 A dryer uses hot air to dry wet feed with 20% moisture content on a wet basis to a final moisture content of 10%, also on a wet basis. To accomplish this, 4225 cfm (cubic feet per minute) of air at 50 °F and 50% relative humidity is preheated to 120 ° F before being fed to the dryer. The air leaves the dryer at 85 °F. Determine the mass flow rate (lbm/min) of wet solid that can be processed by the dryer.

Practice Problem 5.7 It is desired to extract ethylene glycol (E) from 250 lbm of a feed solution containing 60% (by mass) of ethylene glycol and 40% water (W). The solvent used is 150 lbm of pure furfural (F). The relevant equilibrium ternary phase diagram is given below.

Determine: A. the composition of the mixture. B. the mass and composition of the extract and the raffinate. C. the fraction of the solute recovered (also known as recovery)

Practice Problem 5.8 Oil is extracted from a solid substrate fed to a multistage leaching system at the rate of 700 kg/hr. The solids contain 28% oil, and the rest are insoluble solids. Pure organic solvent is fed in a direction counter to the flow of the solids. The requirements specified for the leaching system are as follows: • The solids leaving the system should contain no more than 40 kg/hr of unextracted oil. • The underflow from each stage has 2 kg inerts/kg solution.

424

5

Mass Transfer

• The mass fraction of the solute in the product overflow solution should be no less than 0.20. Determine: A. B. C. D.

The mass flow rate and composition of the underflow solution. The mass flow rate of the overflow solution. The mass flow rate of the pure solvent required. The number of stages required assuming a straight line equilibrium relationship, y = x.

Practice Problem 5.9 Methane is adsorbed by activated carbon from a hydrocarbon gas mixture at atmospheric pressure and 20 °C containing 2 mole% methane. The following equilibrium data is available, where, PCH4 (kPa) mS/mA(g/g)

20 0.13

40 0.24

60 0.35

80 0.42

100 0.50

PCH4 = partial pr:of CH4 , kPa, and ðmS =mA Þ = g CH4 adsorbed=g AC Determine the Freundlich relationship and the volume (L) of methane adsorbed by 100 g of adsorbent.

Solutions to Practice Problems Practice Problem 5.1 Solution Determine m from the given equilibrium relationship, PAi = 0.137CAi, ) m = 0:137 atm= lbmol SO2 =ft3 soln A. Determine the overall liquid mass transfer resistance using Eq. 5.5 and the known values.

Solutions to Practice Problems

425

1 1 1 = þ = K L kL mk G

1 lbmol 0:985 2 hr - ft lbmol SO2 =ft3 soln 1 þ lbmol atm 0:195 0:137 hr - ft2 ðatmÞ lbmol SO2 =ft3 soln = 38:447 lbmol SO2 =ft3 soln = lbmol=hr‐ft2 )

K L = 0:0260 lbmol=hr‐ft2 = lbmol SO2 =ft3 soln Determine the resistance in the liquid film. 1 = kL

1 lbmol 0:985 hr - ft2 lbmol SO2 =ft3 soln

= 1:015 lbmol SO2 =ft3 soln = lbmol=hr‐ft2 Calculate the percentage of the total (overall) resistance in the liquid film.

%resist:in liq film

1 kL = × 100 1 KL lbmol SO2 =ft3 soln 1:015 lbmol=hr‐ft2 ð100Þ = lbmol SO2 =ft3 soln 38:447 lbmol=hr‐ft2 = 2:64%

Note: The result obtained indicates that approximately 97% of the total resistance to mass transfer is in the gas film, implying a gas-phase controlled mass-transfer process. B. Calculate PA using Eq. 5.8. PA

= mC AL atm lbmol SO2 ft3 soln = 0:0053 atm

=

Determine PAG - PA .

0:137

0:0385

lbmol SO2 ft3 soln

426

5

PAG - PA

= 45 mm Hg ×

Mass Transfer

1 atm - 0:0053 atm 760 mm Hg

= 0:0539 atm C. Determine C A using Eq. 5.9. C A =

1 atm PAG 45 mm Hg × 760 mm Hg = 0:4322 lbmol SO2 =ft3 soln = atm m 0:137 lbmol SO2 ft3 soln

Calculate CA - CAL . CA - C AL

lbmol SO2 lbmol SO2 - 0:0385 ft3 soln ft3 soln = 0:3937 lbmol SO2 =ft3 soln

= 0:4322

Alternately, CA - CAL can be calculated using Eq. 5.7, that is, N A = K G PAG- PA = K L C A - C AL . However, this approach requires the calculation of KG first using Eq. 5.4, that is, K1G = k1G þ kmL . Obviously, this would be a more laborious process. D. To calculate PAG - PAi, and CAi - CAL, first calculate the molar flux using Eq. 5.7 and the result for C A - C AL obtained in the solution to part “C.” NA

= K L CA - CAL lbmol 2 = 0:0260 hr‐ft lbmol SO2 ft3 soln = 0:0102 lbmol=hr‐ft2

0:3937

lbmol SO2 ft3 soln

Determine PAG - PAi, and CAi - CAL using Eq. 5.3. N A = k G ðPAG - PAi Þ = k L ðCAi - C AL Þ ) lbmol 0:0102 NA hr - ft2 = 0:0523 atm = PAG - PAi = lbmol kG 2 hr 0:195 - ft atm lbmol 0:0102 NA hr - ft2 = 0:0104 lbmol SO =ft3 soln = CAi - C AL = 2 lbmol kL 2 0:985 hr - ft lbmol SO2 ft3 soln

Solutions to Practice Problems

427

Practice Problem 5.2 Solution Draw the schematic diagram for the process with nomenclature consistent with that used in Fig. 5.3.

A. Calculate the cross-section area of the tower. ACS =

π ð0:85 mÞ2 = 0:5674 m2 4

Calculate the molar flux of the gas entering the tower on a solute-free basis.

GS = G1 ð1 - ya1 Þ =

kmol mix s 0:5674 m2

0:0045

ð1 - 0:06Þ

kmol air kmol mix

= 0:0074 kmol air=s‐m2 Calculate the molar flux of amine solvent entering the absorption tower using its molecular weight and cross section area of the tower. Since pure solvent is entering the tower, LS = L1 and XA2 = 0.

428

5

m_ a min e LS = L1 = M a min e × Acs

0:8

= 32

Mass Transfer

kg s

kg ð0:5674 m2 Þ kmol

= 0:0441 kmol=s‐m2 Calculate YA1 and YA2 using Eq. 5.11. yA1 0:06 = = 0:064 1 - yA1 1 - 0:06 yA2 0:0125 = = 0:0127 Y A2 = 1 - yA2 1 - 0:0125

Y A1 =

Calculate the mole fraction of carbon dioxide in the amine leaving the tower using Eq. 5.10 after first calculating the mole ratio, moles of carbon dioxide to moles of amine in the liquid using the material balance equation (Eq. 5.16).

X A1 =

xA xA1 ) 0:0086 = ) xA1 = 0:0085 1 - xA 1 - xA1

B. Determine the exit mole ratio of ammonia solute to water in equilibrium X A1 with the entrance mole ratio of ammonia to air (YA1) using the given equilibrium relationship. Y A1 = 5:95X A1 ) 0:064 Y X A1 = A1 = = 0:0108 5:95 5:95 Calculate (LS/GS)min using Eq. 5.18. LS GS

= min

ðY A1 - Y A2 Þ 0:064 - 0:0127 = = 4:75  0:0108 - 0 X A1 - X A2

Calculate (LS/GS)actual by substituting the known values.

Solutions to Practice Problems

LS GS Calculate the ratio

429

= actual

LS GS actual =

0:0441 kmols‐mamine 2 = 5:96 kmol air 0:0074 s‐m2

LS GS min :

Required Ratio =

LS GS actual LS GS min

=

5:96 = 1:25 4:75

Practice Problem 5.3 Solution All the relevant lines/curves have been labeled as shown in the figure.

A. From the diagram, the compositions of feed, distillate, and the bottom product are 0.42, 0.93, and 0.06, respectively, that is, xF = 0:42, xD = 0:93, and xB = 0:06, The y-intercept of the Rmin-line is 0.47. Therefore, from Eq. 5.34,

430

5

Mass Transfer

0:93 xD = 0:47 ) = 0:47 ) Rmin = 0:98 Rmin þ 1 Rmin þ 1 B. Extend the rectification line to the y-axis as shown in the figure. This line is the Ractual-line. From the diagram, the y-intercept of the Ractual-line is 0.32. Therefore, from Eq. 5.34, 0:93 xD = 0:32 ) = 0:32 ) Ractual = 1:91 Ractual þ 1 Ractual þ 1 Therefore, the required ratio of actual reflux to minimum reflux is Ratio =

Ractual 1:91 = = 1:95 0:98 Rmin

C. From the figure, determine the slope of the q-line. Slopeq - line =

Δy 28 units = = - 0:80 Δx - 35 units

From Eq. 5.43, slope of q‐line =

q = - 0:80 ) q = 0:44 q-1

From Fig. 5.7, since 0 < q < 1, the feed condition is liquid–vapor mixture. D. From material balance for the column, Eq. 5.33 is

ynþ1 =

Ln D x þ x and Ln þ D = V nþ1 Ln þ D n Ln þ D D

Equation 5.34 for the operating line for the rectification section is ynþ1 =

R x x þ D Rþ1 n Rþ1

ð5:34Þ

Assuming constant molal overflow, the suffixes can be dropped, the preceding equations can be combined to obtain the following result.

Solutions to Practice Problems

431

L D R x xþ xþ D ) x = Rþ1 V Ln þ D D Rþ1 1:91 L Ractual = = 0:66 = Ractual þ 1 1:91 þ 1 V y=

E. The slope of the operating line for the stripping section can be obtained from the diagram and then the material balance relationships (Eqs. 5.36, 5.37, and 5.38) for the stripping section can be used to obtain the following results and eventually the ratio, V′/B. V0 þ1 V þB Δy 16 units L = B 0 ) = = 1:78 = 0 = Slope = 0 Δx 9 units V L -B V B V0 V0 V0 þ 1 = ð1:78Þ ) 0:78 =1 ) B B B 0

0

V0 = 1:28 B

Practice Problem 5.4 Solution First, calculate the mass of completely dry solid, designated as “dry solid.” mS = ð5 kg wet solidÞ 0:90

kg dry solid kg wet solid

= 4:5 kg dry solid Express the initial moisture content in terms of kg H2O/kg dry solid. Therefore, 5 kg wet solid 0:10 kg H2 O 4:5 kg dry solid kg wet solid H = 0:1111 kg 2 O=kg dry solid

Xi =

Calculate the constant drying rate using Eq. 5.50a. Substitute Xf in place of Xc

432

5

Rc =

Mass Transfer

ms X i - X f At c

kg H2 O kg H2 O - 0:05 kg dry solid kg dry solid = ð1:8 m2 Þð4:8 hrsÞ 2 = 0:0318 kg H2 O=m  hr ð4:5 kg dry solidÞ 0:1111

Practice Problem 5.5 Solution A. Using the given information and Eq. 5.50a for constant drying rate, determine the mass of the dry solid as shown. ms ðX i - X c Þ ) At c Rc At c ms = ðX i - X c Þ lbm H2 O 60 min 8:5 2:25 ft2 0:73 hrs × 2 hr min‐ft = lbm H2 O lbm H2 O 0:35 - 0:14 lbm dry solid lbm dry solid = 3989 lbm dry solid Rc =

B. Calculate the total drying time required using Eq. 5.54.

t total =

ms ARc

ðX i - X c Þ þ ðX c - X e Þ ln

3989 lbm dry solid lbm H2 O 2:25 ft2 8:5 min‐ft2 = 68:91 minð1:1485 hrsÞ =

Xc - Xe Xf - Xe

)

ð0:35 - 0:14Þ þ ð0:14 - 0:04Þ × ln

0:14 - 0:04 0:07 - 0:04

Note: The units for moisture content are lbm H2O/lbm dry solid, and they are not shown for the sake of the brevity of the presentation of the equation.

Solutions to Practice Problems

433

Practice Problem 5.6 Solution A schematic diagram for the process is drawn as shown.

The entering solid has 20% moisture content on a wet basis. Using this information and assuming m_ ws as the mass flow rate of the wet solid fed to the dryer, express the mass flow rate of bone-dry solid in terms of the mass flow rate of the wet solid fed to the dryer. 0:8 lbm bone dry solid m_ ws lbm wet solid hr lbm wet solid = ð0:8m_ ws Þ lbm bone dry solid=hr

m_ bds =

The partially dried solid coming out of the dryer has a moisture content of 10%. Based on this information, express the mass flow rate of the partially dry solid in terms of the mass flow rate of the wet solid fed to the dryer. m_ pds

ð0:8m_ ws Þlbm bone dry solid 1 lbm partially dry solid hr 0:90 lbm bone dry solid = ð0:89m_ ws Þ lbm partially dry solid=hr =

434

5

Mass Transfer

Obtain the properties of moist air by locating the state points of air on the psychrometric chart. State point 1 (50 °F, 50% rh): ω1 = 0:004 lbm H2 O=lbm dry air, v1 = 12:9 ft3 =lbm d a State points 2 and 3 (120 °F, ω1 = ω2 = ω3, sensible heating at const. moist.): ω1 = ω2 = ω3 = 0:004 lbm H2 O=lbm dry air To locate state point 4, move along the constant wet bulb temperature line (because of adiabatic saturation of air) from state point 3 to the given final dry bulb temperature of 85 °F. At state point 4, ω4 = 0:012 lbm H2 O=lbm dry air: Using the specific volume of the air entering the pre-heater, calculate the mass flow rate of dry air, m_ a , which remains constant throughout the processes in the pre-heater and dryer. ft 4225 min V_ 1 = = 327:52 lbm d a= min 3 ft v1 12:9 lbm d a 3

m_ a =

The increase in moisture content of air due to the drying process is m_ a ðω4 - ω3 Þ, where ω3 and ω4 are moisture contents in air before and after the drying process. Calculate the mass of water evaporated and absorbed by air by mass balance for water in the solids and hence calculate the mass flow rate of the wet solid fed to the dryer, m_ ws , by substituting all remaining known values. mass of water vapor absorbed by air = mass of water evaporated from wet solid = mass of wet solid in–mass of partially dry solid out m_ a ðω4 - ω3 Þ

= m_ ws - m_ pds = m_ ws - 0:89m_ ws = 0:11m_ ws m_ a ðω4 - ω3 Þ ) m_ ws = lbm H2 O removed 0:11 lbm wet solid lbm d a lbm H2 O lbm H2 O 0:012 327:52 - 0:004 min lbm d a lbm d a = lbm H2 O removed 0:11 lbm wet solid = 23:82 lbm wet solid= min

Solutions to Practice Problems

435

Practice Problem 5.7 Solution A. Determine the composition of the mixture by using overall and component mass balances. FD represents the feed solution, F represents the furfural solvent, and M represents the mixture. Subscript F represents furfural, subscript W represents water, and subscript EG represents ethylene glycol. Overall mass balance: M = FD þ F = 250 lbm þ 150 lbm = 400 lbm F 150 lbm ) xF,M = = = 0:375 M 400 lbm Mass balance for water: ðFDÞxW,FD þ FxW,F = MxW,M ) ðFDÞxW,FD þ FxW,F ð250 lbmÞð0:40Þ þ ð150 lbmÞð0Þ xW,M = = 0:25 = M 400 lbm xEG,M = 1 - xF,M - xW,M = 1 - 0:375 - 0:25 = 0:375 B. Locate the mixture point, M, on the given ternary phase diagram using the preceding compositions. Then, draw the tie line through the mixture point, M, and determine the locations and compositions of the extract, E, and the raffinate, R, on the equilibrium isotherm as shown. The extract, being rich in the solvent, is located closer to the pure solvent on the equilibrium curve and the raffinate is located on the opposite side. On the diagram, EG represents ethylene glycol to make it distinct from E, which represents the extract, and FD represents the feed to make it distinct from F, which represents furfural.

436

5

Mass Transfer

From the phase diagram, the compositions of the extract and raffinate are yEG,E = 0:19, yW,E = 0:07, yF,E = 0:74 xEG,R = 0:49, xW,R = 0:35, xF,R = 0:16 The amounts of the extract and raffinate can be determined from mass and component balances as shown. Overall mass balance: R = M - E = 400 lbm - E Mass balance for water: EyW,E þ RxW,R = MxW,M ) EyW,E þ ð400 - EÞxW,R = 400xW,M ) 400ðxW,M - xW,R Þ ð400 lbmÞð0:25 - 0:35Þ = = 143 lbm E= yW,E - xW,R 0:07 - 0:35 R = 400 lbm - E = 400 lbm - 143 lbm = 257 lbm C. The fraction of the solute recovered can be calculated as follows: Recovery = 1 -

ðxEG,R ÞðRÞ ð0:49Þð257 kgÞ = 0:1605 =1ðxEG,F ÞðFDÞ ð0:60Þð250 kgÞ

Practice Problem 5.8 Solution Draw the schematic diagram for the system as shown.

Solutions to Practice Problems

437

A. Calculate the mass flow rate and composition of the underflow solution, LN as shown. kg B B BN = 504 kg=hr = N ! kg soln: LN kg B 504 hr = 252 kg:soln ðA þ CÞ=hr LN = kg B 2 kg soln: NN = 2

yN =

40 kg CN hr = = 0:1587 LN 252 kg hr

B. Calculate the mass flow rate of the solution, V1. Mass balance for the solute, C:

CV1 ) V1 kg 156 CV1 hr = 780 kg=hr V1 = = 0:20 0:20 x1 = 0:20 =

C. Overall mass balance for the solution (A + C) will result in the answer for the mass flow rate of the pure solvent, VN+1. L0 þ V Nþ1 = LN þ V 1 ) V Nþ1 = LN þ V 1 - L0 kg kg kg = 252 þ 780 - 196 hr hr hr = 836 kg=hr D. The following values can be obtained from a combination of the schematic diagram, from the preceding calculations, and from the equilibrium relationship, y = x.

438

5

Mass Transfer

xNþ1 = yN = 0:1587 x1 = y0 = 1:0

xNþ1 = 0, x1 = 0:20,

Substitute all the known values into Eq. 5.66 to calculate the number of stages required. xNþ1 - xNþ1 0 - 0:16 log 0:20 - 1:0 x1 - x1 = 2:12 =1 þ N =1 þ 0 - 0:20 - x1 x log log Nþ1 0:16 - 1:0 xNþ1 - x1 log

Practice Problem 5.9 Solution Obtain the log of the given data and include it in the data table. PCH4 (kPa) mS/mA (g/g) log PCH4 log (mS/mA)

20 0.13 1.30 -0.89

40 0.24 1.60 -0.62

60 0.35 1.78 -0.46

80 0.42 1.90 -0.38

100 0.50 2.00 -0.30

Plot logðmS =mA Þ vs: log PCH 4 and obtain the slope of the resulting straight line.

Slope =

- 0:30 - ð - 0:80Þ Δy 1 = 0:833 = ) n = 1:2 = 2 - 1:40 Δx n

Obtain the value of K by taking a point on the graph and substituting the known values into Eq. 5.73.

Solutions to Practice Problems

log

439

mS 1 log PCH4 ) = log K þ n mA log K = log

mS 1 1 ð1:80Þ = - 1:96 log PCH4 = - 0:46 mA 1:20 n

) K = 10 - 1:96 = 0:0110 Using Eq. 5.72 as a template for Freundlich equation and the parameters just obtained, the Freundlich equation applicable in this situation is 1 mS = KC n mA

n = 1:2, K = 0:011

→

1 mS = ð0:011ÞðPCH4 Þ1:2 mA

B. For a component in an ideal gas mixture, mole fraction = pressure fraction. Apply the preceding concept to methane in the hydrocarbon mixture. Note that the total pressure is 1 atm = 101 kPa. Therefore, yCH4 =

PCH4 ) PCH4 = yCH4 P = ð0:02Þð101 kPaÞ = 2:02 kPa P

From the Freundlich equation just derived, determine the mass of the solute (CH4) adsorbed by 100 g of activated carbon (adsorbent/solvent). 1 1 mS = ð0:011ÞðPCH4 Þ1:2 ) mS = ðmA Þð0:011ÞðPCH4 Þ1:2 mA 1

= ð100 gÞð0:011Þð2:02 kPaÞ1:2 = 1:9762 g CH4 adsorbed Calculate the moles of CH4 adsorbed by using its molecular weight. N CH4 =

mCH4 1:9762 g = = 0:1235 mol g M CH4 16 mol

Convert the temperature of the hydrocarbon gas mixture to its absolute value. T = 20 ° C þ 273 = 293 K Calculate the volume of methane adsorbed by using the ideal gas law. PV CH4 = N CH4 RT ) L‐atm ð0:1235 molÞ 0:0821 ð293 KÞ N CH4 RT mol K V CH4 = = 2:9708 L = 1 atm P

440

5

Mass Transfer

References 1. J. Benitez, Principles and Modern Applications of Mass Transfer, 3rd edn. (Wiley, Hoboken, 2017) 2. A.S. Foust, L.A. Wenzel, C.W. Clump, L. Maus, B.L. Anderson, Principles of Unit Operations, 2nd edn. (Wiley, Hoboken, 2008) 3. W.L. McCabe, J.C. Smith, P. Harriott, Unit Operations of Chemical Engineering, 7th edn. (McGraw-Hill, New York, 2004) 4. N.S. Nandagopal, Fluid and Thermal Sciences – A Practical Approach for Students and Professionals (Springer Nature, Cham, 2022) 5. R.E. Treybal, Mass Transfer Operations, 3rd edn. (McGraw-Hill, New York, 2012) 6. J. Welty, G.L. Rorrer, D.G. Foster, Fundamentals of Momentum, Heat, and Mass Transfer, 7th edn. (Wiley, Hoboken, 2017)

Chapter 6

Chemical Reaction Engineering and Kinetics

6.1

Introduction

Chemical reactors form the basis of manufacture and synthesis of products in the chemical industry [1, 2, 4]. For example, ammonia is produced using the ammonia synthesis reactor and polymerization reactors are used in producing polymer resins such as polyethylene. There are two major facets [3] in the subject matter of Chemical Reaction Engineering. 1. Kinetics focuses on the study of reaction mechanisms and in the prediction of the speed of reactions. 2. Reactor design involves the use of tools and equations in the actual design of chemical reactors. For example, the volume of the reactor, a key parameter in reactor design, can be calculated using appropriate design equation.

6.2

Key Definitions in Chemical Reaction Engineering

Reaction Rate: The reaction rate refers to the rate of formation of a product species per unit volume per unit time [2, 3]. It can also be represented as the rate of disappearance of a reactant species per unit volume per unit time. The term ‘volume’ refers to the volume occupied by the reaction mixture consisting of both the reactants and products. In general, the reaction rate depends on the temperature and the composition of the reaction mixture. The defining equation for the reaction rate is (a negative sign is appended in case of reactant species) ri =

1 dN i V dt

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 N. S. Nandagopal, Chemical Engineering Principles and Applications, https://doi.org/10.1007/978-3-031-27879-2_6

ð6:1Þ

441

442

6 Chemical Reaction Engineering and Kinetics

Units :

mol kmol lbmol , , L  s m3  s ft3  sec

Reaction Rate Expression: The reaction rate expression is an equation that represents the rate of formation of a product or the rate of disappearance of a reactant [2, 3, 7]. It is best explained by using a generic example. Consider the simple reaction A→B Assuming an elementary reaction (a reaction where the rate expression is consistent with the reaction stoichiometry), the rate of disappearance of species A can be written as - r A = kC A

ð6:2Þ

In the preceding equation, CA is the concentration of species A in the reaction mixture (typical units: mol/L, kmol/m3, lbmol/ft3). Also, in Eq. 6.2, k is known as the reaction rate constant, which determines the speed of a reaction. It is also known as the reaction velocity constant or more commonly as rate constant. The units of the rate constant depend entirely on the rate expression. It is important to remember that the reaction rate will always have the units “moles/volume. time” regardless of the rate expression. Therefore, the units of the rate constant will have to be consistent to make this happen.

Consider the rate expression in Eq. 6.2, -rA = kCA. The units for the rate constant, k, can be derived as shown here. k

1 - r A mol Ls  mol   s-1 s CA L

Supposing the rate expression is, -rA = kCA0.5, the units for the rate constant, k, can be derived as shown here. k

mol mol0:5 - rA  Ls0:5  0:5  mol0:5  L - 0:5  s - 1 0:5 CA L s ðmol L Þ

Order of a Reaction: The order of a reaction refers to the exponents of the concentrations of species in the rate expression [2, 3]. The reaction order is best explained by considering different rate expressions. 1. -rA = kCA. Since the exponent of CA is one, the reaction is first order with respect to species A. 2. - r A = kC 2A . Since the exponent of CA is two, the reaction is second order with respect to species A.

6.2

Key Definitions in Chemical Reaction Engineering

443

3. -rA = kCACB. Since the exponent of CA is one, the reaction is first order with respect to species A. The reaction is also first order with respect to species B. However, since the sum of the exponents of the concentration terms is two, the overall order of the reaction is 2. 1

4. - r A = kC 2A . Since the exponent of CA is ½, the reaction is half order with respect to species A. Elementary and Non-Elementary Reactions: In elementary reactions the reactants are directly converted to products without the formation of intermediate products. Also, the rate law in elementary reactions is consistent with the coefficients in the stoichiometric reaction equation [2, 3]. An example of an elementary reaction is the reaction of nitric oxide (NO) with chlorine (Cl2) to form nitrosyl chloride (NOCl): 2NO þ Cl2 → 2NOCl The rate expression is -r = k[NO]2[Cl2]. The square parenthesis represents the concentration of the species. In contrast, non-elementary reactions usually involve multiple steps and the formation of intermediate products and there is no correlation between the rate expression and the stoichiometric reaction equation [2, 3]. An example of a non-elementary reaction is the decomposition of acetaldehyde or ethanal (CH3CHO) to methane (CH4) and carbon monoxide (CO): CH3 CHO → CH4 þ CO From experimental studies [7], the rate expression is determined to be: - r = k½CH3 CHO1:5 Homogeneous and Heterogeneous Reactions: In homogeneous reactions all the species in the reaction mixture exist in a single phase, that is, in solid, liquid, or gas phase. All gas phase reactions are homogeneous reactions. In contrast, the species in the reaction mixture can exist in different phases in heterogeneous reactions [1–3]. A good example is combustion of coal. The reactants in coal exist in the solid phase while the reactant oxygen from air is in the gas phase. Units of the Rate Constant in Terms of the Order of the Reaction: Consider the generic rate expression - rA = kC nA . Solve for the rate constant, k, with the appropriate units.

- r A = kC nA

mol - rA mol L ) k = n  mol sn  s - 1 CA L L

1-n

ð6:3aÞ

444

6

Chemical Reaction Engineering and Kinetics

In general, for a reaction with order n, the rate constant, k, will have the units of ðtimeÞ - 1 ðconcentrationÞ1 - n

ð6:3bÞ

Stoichiometry Relationship Between Reaction Rates: The reaction rates of different species in a reaction can be related to each other using the stoichiometric reaction equation as illustrated in the following examples. Consider the reaction, 2A þ B → C: From the stoichiometric equation, it is clear that the rate of consumption of species B is one-half the rate of consumption of species A and the rate of formation of species C is the same as the rate of consumption of B because of the stoichiometric ratio 1:1. The relationship between the reaction rates of this example can be mathematically expressed as follows by using species A as the reference. - rB =

- rA - rB - rA rC = rC ) = = 2 1 2 1

Thus, we can conclude that the reaction rates of the species divided by their stoichiometric coefficients will be equal with the understanding of consumption rates (negative sign) for reactants and production rates for products (positive sign). The relationship between reaction rates can be generalized by considering the following generic stoichiometric reaction equation. b c aA þ bB → cC þ dD ) - r B = ð - r A Þ, r C = ð - r A Þ, a a and - rB rC r d - rA rD = ð - rA Þ ) = = = D a b c d a

ð6:4Þ

Fractional Conversion: Fractional conversion refers to the fraction of the original amount of the reactant species that has been consumed in the reaction [2, 3]. If NA0 is the initial number of moles of A present and NA is the final moles of A, then the fractional conversion of A, XA, can be mathematically expressed as XA =

moles A consumed N - NA = A0 moles A initially present N A0

ð6:5Þ

The preceding equation can also be expressed as N A = N A0 ð1- X A Þ

ð6:6Þ

6.3

Arrhenius Equation – Effect of Temperature on the Rate Constant

N A = N A0 - N A0 X A

445

ð6:7Þ

Equations 6.5, 6.6, and 6.7 can be divided by V, the volume of the reaction mixture to obtain the following useful equations in terms of concentrations (C  N/V ) and conversion. XA =

C A0 - C A C A0

ð6:8Þ

CA = C A0 ð1- X A Þ

ð6:9Þ

C A = C A0 - C A0 X A

ð6:10Þ

Equations 6.5, 6.6, and 6.7 are applicable for batch reactors, where typically an initial charge of reactants undergoes the reaction to produce the products. However, flow reactors (where the reactants are fed to the reactor and the product stream emerges from the reactor) must also be considered in connection with fractional conversion. In flow reactors, the molar flow rate, Fi, is used rather than the moles, Ni, that was used in the case of batch reactors. Thus, FA0 represents the molar flow rate of species A fed to the reactor and FA represents the molar flow rate of species A emerging from the reactor after the reaction has taken place and some amount of A has been consumed. The following equations can be written for flow reactors analogous to Eqs. 6.5, 6.6, and 6.7. XA =

F A0 - F A F A0

ð6:11Þ

F A = F A0 ð1- X A Þ

ð6:12Þ

F A = F A0 - F A0 X A

ð6:13Þ

However, it is important to note that the Eqs. 6.8, 6.9, and 6.10 in terms of concentrations and conversion are applicable to batch as well as flow reactors.

6.3

Arrhenius Equation – Effect of Temperature on the Rate Constant

The reaction rate constant, k, shows a strong dependence on the temperature [3, 6]. The higher frequency of molecular collisions at higher temperatures results in faster reactions and this is reflected in the temperature dependency of the rate constant, which is mathematically represented by the Arrhenius equation (Eq. 6.14).

446

6

Chemical Reaction Engineering and Kinetics - Ea

 k = Ae RT

ð6:14Þ

The following nomenclature is used in Eq. 6.14. The variable A is called the frequency factor and it is related to the frequency of collisions, a characteristic of a given reaction, generally obtained by experimental methods. Ea is called the activation energy of the reaction. The activation energy represents the energy that the reactant molecules in a reaction must possess in order for a reaction to occur and it is independent of temperature. The units for activation energy are typically J/mol or Btu/lbmol. R is the universal gas constant and T is absolute temperature. Since the exponent is negative, a combination of low activation energy and high temperature maximizes the value of the reaction rate constant [2, 3, 6]. Taking natural logarithm Eq. 6.14 results in the following equation. ln k = ln A -

Ea R

1 T

ð6:15Þ

It is clear from Eq. 6.15 that a plot of ln k vs. (1/T ) results in a straight line with slope - Ea =R and intercept ln A. This is illustrated in the plot from the solution to Example 6.1. Example 6.1 The following data of reaction rate constant vs. temperature is available for the firstorder decomposition reaction A → B + C T (°F) k (s-1)

500 0.00008

600 0.00135

700 0.0233

800 0.18

Determine: A. the activation energy for this reaction B. the frequency factor for this reaction Solution A. Convert the given data to ln k and 1/T and obtain the following table: T (°F) T°R 1 ° R-1 T -1 k (s ) ln k

500 960 10.4×10-4

600 1060 9.4×10-4

700 1160 8.6×10-4

800 1260 7.9×10-4

0.00008 -9.43

0.00135 -6.61

0.0233 -3.76

0.18 -1.71

6.3

Arrhenius Equation – Effect of Temperature on the Rate Constant

447

Plot ln k vs. 1/T and determine the slope as shown in the graph below.

Slope =

∘ - 2:0 - ð - 7:0Þ Δy -E = - 31250 R =  a ) = Δx 0:0008∘ R - 1 - 0:00096∘ R - 1 R

 = ð31250 RÞð1:986 E a = ð31250 RÞR ∘

∘

Btu Þ = 62, 063 Btu=lbmol lbmol0 R

B. Determine A by solving Eq. 6.14, for A and substituting the known values at one of the given temperatures. From the data given in the problem statement, when the temperature is 500 °F (960 °R), k = 0.00008 s-1. Therefore,

Ea  RT

A = ke = ð0:00008Þ e

1:986

62063 Btu lbmol Btu × 960 R lbmol - R

= 1:095 × 1010 collisions per sec

Example 6.2 Consider two first-order reactions with identical frequency factors. The activation energy of the first reaction is 140000 J/mol and the activation energy of the second reaction is 14000 J/mol. Using T1 = 200 °C and T2 =300 °C as the reference temperatures, determine the ratio of the rate constants k2/k1 with respect to the reference temperatures in each case and comment on the results. Solution Convert the temperatures to their absolute values.

448

6

T 1 = 273 ° þ 200 ° C = 473 K,

Chemical Reaction Engineering and Kinetics

T 2 = 273 ° þ 300 ° C = 573 K

Apply Eq. 6.15 at temperatures T2 and T1 and divide the resulting equations one by the other to obtain the following equation.

ln

- Ea 1 k2 1 = T k1 T R 2 1

)

- Ea k2 =e R k1

1 T2

- T1

1

Apply the preceding equation to reaction 1. - 140000 J

molð 1 - 1 Þ - Ea 1 k2 J 573 K 473 K ð - 1Þ = e R T 2 T 1 = e 8314molK = 1:01 k1

Apply the equation for (k2/k1) ratio to reaction 2. - 14000 J

1 1 mol - Ea 1 k2 J ð573 K - 473 KÞ ð - 1Þ = e R T 2 T 1 = e 8314molK = 1:00 k1

Comment: There is almost no effect due to temperature increase for the reaction with lower activation energy whereas there is an increase in k value for the reaction with higher activation energy. Therefore, reactions with higher activation energies are more sensitive to temperature changes.

6.4

Constant Volume and Variable Volume Systems

Consider the gas phase reaction, H2 ðgÞ þ I2 ðgÞ → 2HIðgÞ Since two moles of reactants result in two moles of products, the volume of the reaction mixture remains constant as the reaction proceeds. This is because the volume of a gas is directly proportional to the number of moles as per the ideal gas equation, PV = NRT: The volume of the reaction mixture also remains constant for liquid phase reactions. This is because mass is always conserved and density remains constant for liquid-phase reaction mixtures. Thus, liquid phase reactions and gas phase reactions with equal number of reactant and product moles can be considered as constant volume or constant density

6.4

Constant Volume and Variable Volume Systems

449

systems and Eqs. 6.8, 6.9, and 6.10, which were obtained by dividing the moles by constant volume, V, are applicable only for constant volume systems. In such cases, the volume of the reaction mixture is not affected by the conversion. Now, consider the popular gas phase reaction for the synthesis of ammonia. N2 ðgÞ þ 3H2 ðgÞ → 2NH3 ðgÞ: In this case, four (4) moles of reactants result in only two (2) moles of products. Thus, the volume of the reaction mixture decreases as the reaction proceeds. Therefore, the volume of the reaction mixture is affected by the conversion, X. Variable volume systems or variable density systems are handled by using the parameter, ε, which is defined as the fractional change in volume in going from zero conversion (Xi = 0) to complete conversion (Xi = 1). Using reactant A as the reference species, the equation for the parameter, ε, can be mathematically expressed as shown in Eq. 6.16 [2, 3, 6]. εA =

V XA = 1 - V XA = 0 V XA = 0

ð6:16Þ

Equation 6.16 can be applied to the ammonia synthesis reaction as shown here. Again, the volume is directly proportional to the number of moles for gaseous systems. εA =

V XA = 1 - V XA = 0 2 - 4 = = - 0:50 V XA = 0 4

Since εA represents the fractional change in volume as the reaction proceeds from zero conversion to complete conversion, the volume of the reaction mixture, V, can be expressed in terms of the initial volume, V0, and the conversion, XA, as shown in the following equation. V = V 0 ð1 þ ε A X A Þ

ð6:17Þ

Equations 6.6 and 6.17 can be combined as shown to obtain the following result for variable volume (or variable density) systems. CA =

N ð1 - X A Þ 1 - XA NA = CA0 = A0 V 1 þ εA X A V 0 ð1 þ ε A X A Þ

) C A = C A0

1 - XA 1 þ εA X A

ð6:18Þ

Thus, only Eq. 6.18 can be used to express the molar concentration as a function of fractional conversion for variable volume systems.

450

6.5

6

Chemical Reaction Engineering and Kinetics

List of Useful Integrals in Reactor Design

Application of the design equations for reactor design sometimes involves integration of functions involving fractional conversion, X, and the fractional change in volume, ε. Integrals commonly used in reactor design [2] are listed here for quick reference and use. X

dX 1 = ln 1-X 1-X

ð6.I-1Þ

0

X

dX 1 lnð1 þ εX Þ = 1 þ εX ε

ð6.I-2Þ

1 þ εX 1 - εX dX = ð1 þ εÞ ln 1-X 1-X

ð6.I-3Þ

0

X

0

X

X dX = 2 1 X ð1 - X Þ

ð6.I-4Þ

ð1 þ εÞX 1 þ εX 1 dX = - ε ln 2 1 X 1 X ð1 - X Þ

ð6.I-5Þ

0

X

0

X

0

6.6

ð1 þ εX Þ2 ð1 þ εÞ2 X 2 dX = 2ε ð 1 þ ε Þ ln ð 1X Þ þ ε X þ 1-X ð1 - X Þ2

ð6.I-6Þ

Reactor Types, Design Equations, and Applications

There are three types of ideal reactors [1, 2, 3]: Batch Reactor, Plug Flow Reactor, and Mixed Flow Reactor. In a batch reactor, the reactants are charged and the reaction proceeds after the desired conditions of temperature and pressure are reached. In a batch reactor, the composition of the reaction mixture changes

6.6

Reactor Types, Design Equations, and Applications

451

continuously with time. Thus, a batch reactor is inherently an unsteady state reactor. After allowing sufficient time for the desired conversion of the reactants, the products are removed and separated. However, both the flow reactors operate under steady-state conditions. The plug flow reactor (PFR) is also known as tubular flow reactor (TFR) and the mixed flow reactor (MFR) is also known as a continuous stirred tank reactor (CSTR). The design equations (also known as ‘performance equations’) for reactors are used in sizing the reactors. The design equations are derived from the fundamental material balance equation (Eq. 1.1).

6.6.1

Batch Reactor

A schematic representation of a batch reactor is shown in Fig. 6.1.

6.6.1.1

Design (or Performance) Equation for a Batch Reactor

Start with Apply the fundamental material balance equation (Eq. 1.1) to a batch reactor as shown here.

Fig. 6.1 Batch reactor schematic

452

6

Chemical Reaction Engineering and Kinetics

Rate of reactant input - rate of reactant consumption - rate of reactant output = rate of reactant accumulation within the reactor After the reactor is initially charged, there is no input or output for a batch reactor. Therefore, Rate of reactant consumption = - ðrate of reactant accumulation within the reactorÞ The rate of reactant consumption is the rate of disappearance of A due to reaction in terms of moles of A consumed per unit time. The reaction rate expression represents the moles of a species consumed per unit time per unit volume of the reaction mixture. Therefore, Rate of reactant consumption = ð- r A ÞV Rate of reactant accumulation within the reactor =

dN A dt

Combine the preceding equations to obtain the following results.

ð- r A ÞV = -

d dX dN A = - ½N A0 ð1- X A Þ = N A0 A ) dt dt dt ð- r A ÞV = ðN A0 Þ

dX A dt

Integration of the preceding equation after separating the variables, results in the general design equation for a batch reactor. t = ðN A0 Þ

XA 0

dX A ð - r A ÞV

ð6:19Þ

For constant volume systems, the volume of the reaction mixture remains constant as the reaction progresses and the volume term within the integral sign is independent of the conversion. In such cases, the design equation is XA

XA

dX A = CA0 ð - rA Þ

N t = A0 V 0

dX A ð - rA Þ

ð6:20Þ

0

For variable volume systems, Eqs. 6.17 and 6.19 can be combined to obtain the following general design equation for a batch reactor [2, 3].

6.6

Reactor Types, Design Equations, and Applications

XA

453

XA

dX A = C A0 ð - r A ÞðV 0 Þð1 þ εA X A Þ

t = N A0 0

dX A ) ð - r A Þð1 þ εA X A Þ 0

XA

dX A ð - r A Þð1 þ εA X A Þ

t = C A0

ð6:21Þ

0

Example 6.3 The irreversible gas phase reaction, 2A(g) → B(g), is second order in reactant A. The reaction rate constant is k = 3.59 ft3/lbmol-min. A batch reactor is charged with pure A at 20 psia and 100 °F. Calculate the time required for 80% conversion of A. Solution Convert the reactor temperature to its absolute value T = 460 ° þ 100 ° F = 560 ° R Calculate the initial molar concentration of A using the ideal gas law.  ) PV 0 = N A0 RT N A0 P = C A0 =  = V0 RT

20 psia psia‐ft3 10:73 lbmol‐0 R

= 0:0033 lbmol=ft3 0

560 R

Write down the rate expression for the second-order reaction. Since this is a gas phase reaction with two moles of reactant and one mole of product, it is a variable volume system and hence Eq. 6.18 should be used for CA in the rate expression. 1 - XA 1 þ εA X A

- rA = kC2A = kC 2A0

2

Substitute the preceding result into the general design equation for a batch reactor (Eq. 6.21). t = CA0

XA 0

t=

1 kC A0

dX A = CA0 ð - r A Þð1 þ εA X A Þ XA

0

ð1 þ εA X A Þ dX A ð1 - X A Þ2

XA 0

dX A 1 - XA 2 2 kCA0 ð1þεA X A Þ ð1

þ εA X A Þ

)

454

6

Chemical Reaction Engineering and Kinetics

Substitute the integration result from Eq. 6.I-5 into the preceding equation to obtain the the following expression for t. t=

1 ð1 þ εÞX 1 - εln kC A0 1 - X 1-X

Also, calculate the value of ε, the fractional change in the volume of the reaction mixture using Eq. 6.16 and in the final step substitute all the known values into the preceding equation for t, the time required for the desired conversion. εA =

t=

V XA = 1 - V XA = 0 1 - 2 = = - 0:5 2 V XA = 0

1 ð1 þ εÞX 1 - εln; kCA0 1 - X 1-X 1

t=

3

3:59

ft lbmolmin

0:0033

lbmol ft3

) ð1 - 0:5Þð0:8Þ 1 - ð - 0:5Þln; 1 - 0:8 1 - 0:8

= 237 minð ≈ 4 hrsÞ

6.6.2

Mixed Flow Reactor

A mixed flow reactor is also known as continuous stirred tank reactor (CSTR). Figure 6.2 is a schematic diagram of a CSTR. The most important feature of a CSTR is that the contents of the tank are well-mixed and therefore the conditions at the exit of the reactor are the same as those in the tank.

Fig. 6.2 Schematic diagram of a mixed flow reactor (CSTR)

6.6

Reactor Types, Design Equations, and Applications

6.6.2.1

455

Design Equation for a Mixed Flow Reactor

The following nomenclature is used in the design and analysis of a CSTR. CA0 = concentration of reactant A in the entering stream (mol/L, lbmol/ft3, kmol/m3) FA0 = molar flow rate of reactant A entering the reactor (mol/s, lbmol/hr, kmol/s) ν0 = volume flow rate of the stream entering the reactor (L/s, ft3/hr, m3/s) V = volume of the reactor (L, ft3, m3) CA = concentration of reactant A in the exit stream and in the reactor FA = molar flow rate of A leaving the reactor Start with the fundamental material balance equation (Eq. 1.1). Rate of reactant input - rate of reactant consumption - rate of reactant output = rate of reactant accumulation within the reactor Since a CSTR operates under steady-state conditions, there will be no accumulation of reactant within the reactor. Therefore, the following equation can be written in terms of molar rates. Rate of reactant input - rate of reactant consumption = rate of reactant output F A0 - ð- r A ÞV = F A0 ð1- X A Þ The preceding equation is simplified and solved for V, the volume of the reactor. V=

F A0 X A ð - rA Þ

ð6:22Þ

The space time, τ, for flow reactors is defined as the ratio of the volume of the reactor to the volumetric flow rate of the feed. τ=

V v0

ð6:23Þ

Divide both sides of Eq. 6.22 by the volumetric flow rate of the feed, v0. F A0 v0 X A V F A0 X A V= ) = v0 ð - rA Þ ð - rA Þ

In the preceding equation, the ratio of the molar flow rate of reactant A entering the reactor to the volume flow rate of the feed is the molar concentration of A entering the F V = τ: Substitute these results into the preceding reactor, that is, A0 = C A0 and v0 v0 equation to obtain another form of the design equation for a CSTR [2, 3].

456

6 Chemical Reaction Engineering and Kinetics

τ=

C A0 X A ð - rA Þ

ð6:24Þ

Note: The space velocity is defined as the ratio of the volume flow rate of the feed at standard temperature and pressure (STP) to the volume of the reaction mixture, that is, SV = v0 (STP)/V. Example 6.4 The liquid phase reaction, A + B → C, is first order in reactant A since a large excess of reactant B is used. The rate constant is 2.76 hr-1. 13,500 lbm/hr of a feed stream consisting of 10% by mass of reactant A is fed to an isothermal mixed flow reactor. The density of the feed stream is 55 lbm/ft3 and the molecular weight of reactant A is 45 lbm/lbmol. Determine the volume of the reactor in gallons required to achieve 80% conversion of reactant A. Solution Calculate the volume flow rate of the feed stream using mass flow rate and density of the feed. v0 =

m_ feed 13500 lbm hr = = 245:45 ft3 =hr ρfeed 55 lbm 3 ft

Calculate the molar flow rate of reactant A by using its mass percentage in the feed and its molecular weight. F A0 =

lbm m_ A0 ð0:10Þ 13500 hr = = 30 lbmol=hr lbm MA 45 lbmol

Write down the rate expression for this first-order reaction and substitute for CA using Eq. 6.9, that is, [CA = CA0(1 - XA)]. - r A = kC A = kCA0 ð1- X A Þ Calculate the volume of the reactor by using the design equation (Eq. 6.22) for a CSTR and substitute for -rA from the preceding equation. Also note that for flow reactors, CFA0 = v0 . Therefore, A0

6.6

Reactor Types, Design Equations, and Applications

V=

V=

F A0 X A 1 F A0 F A0 X A = = ð - r A Þ kC A0 ð1 - X A Þ k C A0 XA v0 = k 1 - XA

245:45

ft3 hr

2:76 hr - 1

457

XA 1 - XA

)

0:80 = 355:72 ft3 1 - 0:80

Convert the calculated volume to gallons. V = 355:72 ft3 × 7:4805

gallons = 2661 gallons ft3

Example 6.5 The liquid phase reaction, A þ B → C, ΔH R = 9, 600 Btu=lbmol A takes place in an adiabatic CSTR with large amounts of excess reactant B in the feed stream. 12,000 lbm/hr of a feed stream at 100 °F, consisting of 20% by mass of reactant A is fed to the reactor where the desired conversion is 70% of reactant A (molecular weight 40 lbm/lbmol). The specific heat of the reaction mixture is 1.25 Btu/lbm-°F. Determine the temperature of the stream leaving the reactor. Solution Calculate the molar flow rate of reactant A by using its mass percentage and its molecular weight. ð0:20Þ 12000 lbm m_ hr = 60 lbmol=hr N_ A0 = A0 = lbm MA 40 lbmol Calculate the lbmol/hr of A reacting by using the conversion. lbmol ð0:70Þ = 42 lbmol=hr N_ A,reacting = N_ A0 X A = 60 hr Calculate the rate of heat absorbed by the endothermic reaction by using the heat of the reaction. Q_ reaction = N_ A, reacting ðΔH R Þ lbmol A reacting hr = 403,200 Btu=hr =

42

9600

Btu lbmol A reacting

Since the reactor is operating under adiabatic conditions, there is no heat transfer from an external heat source. Therefore, the source of heat for this endothermic reaction is the heat of the reaction mixture itself resulting in temperature decrease of the reaction mixture. Calculate the exit temperature of the reaction mixture by using energy balance.

458

6

Chemical Reaction Engineering and Kinetics

Rate of heat absorbed by the endothermic reaction = rate of thermal energy decrease of the reaction mixture Btu = m_ mix cP, mix ΔT mix ) hr Btu Btu 403200 403200 hr = hr ΔT mix = lbm Btu m_ mix cP, mix 1:25 12000 hr lbm‐0 F

403200

= 26:88 ° F

Calculate the temperature of the exit stream from the reactor. T exit = T inlet - ΔT mix = 100 ° F - 26:88 ° F = 73:12 ° F Example 6.6 Consider the following liquid phase reversible reaction: k1

2A ⇄ B: k2

The reaction is elementary, that is, second order with respect to species A and first order with respect to species B. The rate constants are: k1 = 6.5 L/mol.min and k2 = 2.3 min-1. 26.7 kg/min of pure A (molecular weight 36 g/mol) is fed to a steady-state CSTR. The density of A is 885 kg/m3. Calculate the volume of the reactor (in liters) required to accomplish 90% conversion of A. Solution Since this is a liquid phase reaction, the volume of the reaction mixture remains constant. Write down the rate expression for this reversible reaction based on second order for the forward reaction (species A) and first order for the reverse reaction (species B) and use Eq. 6.9, that is, CA = CA0(1 - XA) to obtain the following result for the rate expression. - r A = k1 C 2A - k 2 C B = k 1 C 2A0 ð1 - X A Þ2 - k2 CB From the stoichiometric reaction equation, Moles of B formed = 0.5 (moles of A consumed) ) N B = 0:5ðN A0 X A Þ )

N A0 NB = 0:5 X A ) CB = 0:5C A0 X A V V

Substitute for CB from the preceding equation into the rate expression to obtain the following equation for the rate expression.

6.6

Reactor Types, Design Equations, and Applications

459

- r A = k1 C 2A0 ð1 - X A Þ2 - 0:5k2 CA0 X A Calculate the molar flow rate of the feed consisting of pure A by using the mass flow rate and molecular weight of A. F A0 =

kg 1000 g m_ A0 26:7 min × kg = = 741:7 mol= min g MA 36 mol

Calculate the volume flow rate of the feed using the mass flow rate and density of the feed. v0 =

kg m_ feed 26:7 min = = 0:0302 m3 = min ρfeed 885 mkg3

Calculate the initial concentration of A as shown. C A0 =

741:7 mol F A0 min = = 24:56 mol=L m3 v0 0:0302 min × 1000 mL3

Calculate the volume of the reactor by using the design equation (Eq. 6.22) for a CSTR and substitute for the rate expression, -rA, from the equation derived earlier. V=

=

6.6.3

F A0 X A F A0 X A = ð - r A Þ k1 C2A0 ð1 - X A Þ2 - 0:5k 2 C A0 X A mol 741:7 × 0:90 min = 48:41 L L mol 2 2 ð6:5 Þð24:56 Þ ð1 - 0:9Þ mol  min L mol - ð0:5Þð2:3 min - 1 Þð24:56 Þð0:9Þ L

Plug Flow Reactor

The plug flow reactor consists of a tubular structure through which the reactants are introduced. Hence, it is also known as a tubular flow reactor. The reaction progresses as the reaction mixture moves through the tube, maintaining uniform conditions at any given section of the reactor. Hence, the conversion increases along the length of the reactor. A schematic diagram of a plug flow reactor is shown in Fig. 6.3.

460

6

Chemical Reaction Engineering and Kinetics

Fig. 6.3 Schematic diagram of a plug flow reactor

6.6.3.1

Design Equation for Plug Flow Reactor

Start with the fundamental material balance equation (Eq. 1.1). Rate of reactant input - rate of reactant consumption - rate of reactant output = rate of reactant accumulation within the reactor Since a PFR operates under steady-state conditions, there will be no accumulation of reactant within the reactor. Therefore, the following equation can be written in terms of molar rates. Rate of reactant input = rate of reactant output þ rate of reactant consumption Apply the preceding material balance equation to the differential element of length dx and volume dV shown in Fig. 6.3. F A = ðF A þ dF A Þ þ ð- r A ÞV However, FA = FA0(1 - XA) Differentiate the preceding equation to obtain dFA = (-FA0)dXA Substitute the preceding result into the material balance equation. ðF A0 ÞdX A = ð- rA ÞdV ) dV =

ðF A0 ÞdX A ð - rA Þ

Integration of the preceding equation results in the design equation for a PFR. V = F A0

XA 0

dX A ð - rA Þ

ð6:25Þ

Divide both sides of Eqn. 6.25 by the volume flow rate, v0, to obtain another form of the design equation for a PFR [2, 3]. Note that vV0 = τ and FvA00 = CA0 :

6.6

Reactor Types, Design Equations, and Applications

τ=

V F = A0 v0 v0

XA 0

dX A = C A0 ð - rA Þ

461 XA 0

dX A ð - rA Þ

ð6:26Þ

Example 6.7 Consider the following gas phase decomposition reaction: 2AðgÞ → BðgÞ þ C ðgÞ The reaction has second-order kinetics in species A (molecular weight 44 kg/kmol), and it takes place in a plug flow reactor at 200 kPa and 180 °C. The reaction rate constant is 36.7 m3/kmol.s. The gaseous feed consists of 0.35 kg/s of pure A, also at reactor conditions. Determine the volume of the reactor required for 70% conversion of A. Solution The volume of the reaction mixture will remain constant in this gas phase reaction since the number of moles of reactants and products are equal at 2 moles each. Calculate the molar flow rate of the feed consisting of pure A by using the mass flow rate and molecular weight of A. F A0 =

0:35 kgs m_ A0 = = 0:0079 kmol=s kg MA 44 kmol

Convert the temperature of the reactor (as well as that of the feed) to its absolute value. T = 273 ° + 180 ° C = 453 K Calculate the initial molar concentration of A using the ideal gas law. CA0 =

Note on units:

P = RT

200 kPa = 0:0531 kmol=m3 kJ 8:314 ð453 KÞ kmol  K

k mN2 kPa 1   kN  m m3 kJ

Write down the rate expression for this second-order reaction in species A. and use Eq. 6.9, (applicable for constant volume systems), that is, CA = CA0(1 - XA), to obtain the following result for the rate expression - r A = kC 2A = kC 2A0 ð1 - X A Þ2 Substitute the preceding rate expression into the design equation (Eq. 6.25) for a PFR and simplify. Also, use the integration result from Eq. 6.I-4, that is, X 0

dX ð1 - X Þ2

=

X 1-X

462

6

V = F A0

XA 0

V=

F A0 kC 2A0

dX A = F A0 ð - rA Þ

XA 0

Chemical Reaction Engineering and Kinetics

dX A ð1 - X A Þ

2

=

XA 0

dX A 2 kC A0 ð1 - X A Þ2

F A0 kC 2A0

)

XA 1 - XA

Substitute all the known values into the preceding equation for V.

V=

F A0 kC2A0

= 1:15 m

6.6.4

0:0079

XA = 1 - XA

m3 5:7 kmol:s

kmol s 0:0531kmol 3

2

0:7 1 - 0:7

m

3

Comparison of Volumes of Mixed Flow Reactor and Plug Flow Reactor

The design equation for calculating the volume of a CSTR (Eq. 6.22) is area of the rectangle with sides V = ðF-A0rXAAÞ ) FVA0 = - r1A ðX A Þ = -

1 rA

andðX A Þ on a graph of -

1 rA

vs: X A as shown in Fig. 6.4.

Fig. 6.4 CSTR and PFR volume comparison

6.6

Reactor Types, Design Equations, and Applications

463

However, the design equation for calculating the volume of a plug flow reactor X dX A X A dX A V (Eq. 6.25) is V = F A0 0 A ð rA Þ ) F A0 = 0 ð - rA Þ = area under the curve on the plot of - r1A vs:X A as shown in Fig. 6.4. Thus, for a given set of conditions, a CSTR will always have a greater volume requirement compared to a PFR [3]. Example 6.8 The liquid phase reaction 2A → B is second order in reactant A. The rate constant is 38.7 ft3/lbmol ‐ hr. 1500 lbm/hr of a feed stream consisting of pure A is fed to isothermal reactors, first to a CSTR and then separately and independently to a PFR. The density of the feed stream is 52 lbm/ft3 and the molecular weight of reactant A is 35 lbm/lbmol. It is desired to achieve 80% conversion of reactant A. Compare the volumes of CSTR and PFR reactors required to obtain the desired conversion. Solution Calculate the volume flow rate of the feed stream by using the density of the feed. v0 =

m_ feed 1500 lbm hr = = 28:85 ft3 =hr lbm ρfeed 52 3 ft

Calculate the molar flow rate of reactant A from the mass flow rate and molecular weight. 1500 lbm m_ hr F A0 = A0 = = 42:86 lbmol=hr lbm MA 35 lbmol Calculate the molar concentration of species A in the feed using the molar flow rate and the volume flow rate. C A0 =

F A0 42:86 lbmol hr = = 1:486 lbmol=ft3 v0 ft3 28:85 hr

Write down the rate expression for this second-order liquid phase, constant volume reaction and substitute for CA using Eq. 6.9, that is, [CA = CA0(1 - XA)]. - r A = kC 2A = kC 2A0 ð1 - X A Þ2 Volume of CSTR Required Calculate the volume of a CSTR required by using the design equation (Eq. 6.22) for a CSTR, substitute for -rA from the preceding equation, and then substitute all the other known values.

464

6

Chemical Reaction Engineering and Kinetics

F A0 X A F A0 X A = ð - r A Þ kC 2A0 ð1 - X A Þ2 lbmol 42:86 ð0:80Þ hr = = 10:03 ft3 ft3 2 ð1 - 0:80Þ2 38:7 1:486 lbmol ft3 lbmolhr

V CSTR =

Volume of PFR Required Calculate the volume of a PFR required by using the design equation (Eq. 6.25) for a PFR, substitute for -rA from the rate expression, use the result from Eq. 6.I-4 from X

the list of integrals, that is, 0

dX ð1 - X Þ2

=

X 1-X ,

and then substitute all the other known

values. V PFR = F A0

XA 0

V PFR =

dX A = F A0 ð - rA Þ

XA F A0 = kC 2A0 1 - X A

XA

dX A 2

=

F A0 kC2A0

2 0 kC A0 ð1 - X A Þ lbmol 42:86 hr ft3 1:486 lbmol 38:7 ft3 lbmolhr = 2:01 ft3

XA

dX A

0

ð1 - X A Þ2

2

0:80 1 - 0:80

)

As expected, under the same operating conditions, the required volume of a CSTR is much greater than the required volume of a PFR.

6.6.5

CSTRs in Series

Consider n equal-sized CSTRs arranged in series as shown in Fig. 6.5. The concentration of the reactant entering the first reactor is C0. The subscripts denote the concentration of the reactant leaving a particular reactor and entering the subsequent reactor. For example, the concentration of the reactant leaving the first reactor and entering the second reactor is C1, and the concentration of the reactant leaving the second reactor and entering the third reactor is C2, and so on. The concentration of

Fig. 6.5 n equal sized CSTRs in series

6.6

Reactor Types, Design Equations, and Applications

465

the reactant leaving the (n - 1)th reactor and entering the nth reactor is Cn–1 and the concentration of the reactant leaving the nth reactor Cn. Similarly, the subscripts for the conversions indicate the conversion accomplished in the respective reactors. X1 is the conversion achieved in in the first reactor, X2 is the conversion achieved in the second reactor, and so on until Xn is the conversion achieved in in the nth reactor. Equation 6.9, that is, CA = CA0(1 - XA), can be sequentially applied to each reactor. Therefore, after dropping the subscript for the species, A, the following equations can be obtained. C1 = C 0 ð1 - X 1 Þ, C 2 = C 1 ð1 - X 2 Þ, . . . . . . :, C n = Cn - 1 ð1 - X n Þ Each reactor has a volume, V, as shown in Fig. 6.5 and the steady-state volume flow rate through the entire system is v. Each reactor has an identical space time, that is, τ = Vv Apply the design equation for CSTR (Eq. 6.24) for the first reactor and consider a first-order reaction. C A0 X A C0 X 1 C0 X 1 ) τ= ) = ð - rA Þ kC 1 kC0 ð1 - X 1 Þ X1 1 C0 kτ = ) 1 þ kτ = = , ðbecause C 1 = C0 ; ð1 - X 1 ÞÞ 1 - X1 1 - X 1 C1 τ=

Equation 6.24 can be applied in a similar fashion for the second reactor and then all the subsequent reactors resulting in the following set of equations. 1 C1 = 1 - X 2 C2 1 C2 1 þ kτ = = 1 - X 3 C3 1 C3 1 þ kτ = = ::::: 1 - X 4 C4 Cn - 1 1 = 1 þ kτ = 1 - Xn Cn 1 þ kτ =

Therefore, C0 C0 C1 C2 C C = : : . . . . . . . . . . . . . . . . . . . . . . . . n-2 : n-1 = ð1 þ kτÞn Cn C1 C2 C3 C n-1 C n

ð6:27Þ

Solve the preceding equation for τ, the space time in each reactor. τ=

1 k

C0 Cn

1 n

-1

ð6:28Þ

Multiply Eq. 6.28 by n to obtain the total space time in the n reactors [4, 5].

466

6

τn =

n k

Chemical Reaction Engineering and Kinetics

C0 CN

1 n

-1

ð6:29Þ

Example 6.9 Component A undergoes the following first-order reaction in the liquid phase in a CSTR: A → B. The initial concentration of A is 0.85 mol/L. The reaction rate constant is 0.190 s-1. The volume flow rate of liquid A fed to the reactor is 2.5 L/s. Determine A. the volume of a single CSTR required to achieve 90% conversion of A. B. the volume of each CSTR, when two CSTRs are used in series. Comment on the results for part A and part B. Solution The rate expression in this case with 90% conversion of reactant A is - rA = kC A = kC A0 ð1 - X A Þ mol = ð0:190 s - 1 Þ 0:85 ð1 - 0:9Þ = 0:0161 mol=L  s L A. Calculate the required volume of a single CSTR using the design equation for a CSTR (Eq. 6.24). V CA0 X A CA0 X A ) = ) ð - rA Þ v0 ð - r A Þ L mol 2:50 0:85 ð0:90Þ v0 C A0 X A s L = V= = 118:79 L mol ð - rA Þ 0:0161 Ls τ=

B. Calculate the volume of each reactor for two identical CSTRs in series by using Eq. 6.28. 1 τ= k v0 V= k

C0 Cn

1 n

V 1 -1 ) = v0 k

C0 C 0 ð1 - X overall Þ

1 n

C0 Cn

1 n

-1 )

L s -1 = 0:19 s - 1 2:50

1 1 - 0:9

1 2

- 1 = 28:45 L

Practice Problems

467

Comment: Under the same conditions of required conversion, volume flow rate, and rate constant, having two identical CSTRs in series significantly reduces the volume required for each reactor compared to having a single CSTR [4, 5]. An infinite number of identical CSTRs in series approximates a plug flow reactor in operation and reaction profile. Under same operating conditions, the volume of a plug flow reactor is significantly less than the volume of a CSTR. Thus, having multiple identical CSTRs in series significantly reduces the required volume of each reactor. However, a detailed economic study is necessary to determine the optimum number of CSTRs required in series.

Practice Problems Practice Problem 6.1 For the reaction, A + B → C, the reaction rate constant is 3.85 × 10-4 mol-1.s-1 at 285 °C and the rate constant is 1.75 × 10-2 mol-1.s-1 at 366 °C. For this reaction, determine: A. the activation energy. B. the frequency factor.

Practice Problem 6.2 The following second-order liquid phase reaction takes place in a batch reactor, where the initial charge is 780 kg of A (molecular weight, 58 kg/kmol). 2AðlÞ → BðlÞ þ C ðlÞ The density of the reactant liquid is 885 kg/m3. The reaction rate constant is k = 0.0062 m3/kmol.min. Determine the volume of the reactor required if it takes 1.35 hours to achieve 75% conversion of A.

Practice Problem 6.3 The first-order liquid phase reaction A → B takes place in a CSTR with a volume of 155 L. The feed consists of pure A at the rate of 40 L/min. Determine the value of the rate constant required to achieve 85% conversion of reactant A.

468

6

Chemical Reaction Engineering and Kinetics

Practice Problem 6.4 A gas phase homogeneous elementary reaction, A(g) → 2B(g) takes place in a plug flow reactor with a total length of 13.8 ft. The ID of this tubular reactor is 16.88 in. The conditions within the reactor are 285 °F and 55 psia and the rate constant is 3.1 min-1. The feed consists of pure A with a volume flow rate of 16 cfm at reactor conditions. Determine fractional conversion of A achieved in this reactor.

Practice Problem 6.5 The single CSTR available in Example 6.9 is now required to handle an increased feed flow rate of 3.1 L/s and the revised required conversion of A is 95%. It is proposed to handle this new situation by using an additional CSTR in series with the existing CSTR. For the additional CSTR, determine A. the conversion achieved. B. the volume required.

Practice Problem 6.6 Five equal sized CSTRs are used in achieving an overall 98% conversion of species A undergoing a first-order reaction with a rate constant, k = 0.55 s-1. The volume flow rate of the feed through the system is 10 L/s. Determine the volume of each reactor in the system.

Solutions to Practice Problems Practice Problem 6.1 Solution A. Convert the temperatures to their absolute values. T 1 = 273 ° þ 285 ° C = 558 K,

T 2 = 273 ° þ 366 ° C = 639 K

Apply Eq. 6.15 at temperatures T2 and T1 and divide the resulting equations one by the other to obtain the following equation.

Solutions to Practice Problems

ln

469

k2 1 - Ea 1 = k1 T2 T1 R

) Ea =

R × ln

k2 k1

-

1 T2

1 T1

8:314 =

kJ kmol  K

0:0175 mol - 1  s‐1 0:000385 mol - 1  s‐1 - 6391 K

ln 1 558 K

= 1:397 × 105 kJ=kmol B. Determine A by using Eq. 6.15 and substituting the known values at one of the given temperatures. ln k 2 = ln A -

Ea R

1 T2

) A=e

ln ð0:0175Þþ1:397 × 105 kJ kmol × 639 8:314

A=e

kJ kmolK

Ea ln k 2 þRT 

2

)

K

= 4:603 × 109

Practice Problem 6.2 Solution Start with the design equation for a batch reactor (Eq. 6.19) and simplify using the fact that in a liquid phase reaction, the volume of the reaction mixture is independent of the conversion. Hence, V can be taken outside the integral sign and N A0 = CA0 : V XA

t = ðN A0 Þ

0

t = CA0

XA 0

N dX A ) t = A0 V ð - r A ÞV

XA 0

dX A ) ð - rA Þ

dX A ð - rA Þ

Write down the rate expression for this second-order reaction and combine it with Eq. 6.9 to obtain the following result. - r A = kC 2A = kC 2A0 ð1 - X A Þ2 Substitute the preceding result into the design equation and obtain an equation X

for the volume of the reactor by using the integration result from Eq. 6.I-4 dX ð1 - X Þ2

=

0 X 1-X

as shown.



470

6

XA t = C A0 0

1 = 2 2 kCA0 kC A0 ð1 - X A Þ dX A

)V=

ktN A0 XA

dX A 0

ð1 - X A Þ2

=

XA 0

Chemical Reaction Engineering and Kinetics

V = 2 kN A0 ð1 - X A Þ dX A

XA 0

dX A ð1 - X A Þ2

ktN A0 ðktN A0 Þð1 - X A Þ )V= XA XA 1 - XA

Calculate the initial moles of A using the molecular weight of A. N A0 =

mA0 780 kg = = 13:448 kmol kg MA 58 kmol

Substitute all the known values into the equation for the volume of the reactor. V=

ðktN A0 Þð1 - X A Þ XA 3

=

m 0:0062 kmol min

1:35 hr ×

60 min hr

ð13:448 kmolÞ × ð1 - 0:75Þ

0:75

= 2:251 m3

Practice Problem 6.3 Solution Calculate the space time for this reactor using Eq. 6.23. τ=

155 L V = L = 3:875 min v0 40 min

Use the design equation for a CSTR (Eq. 6.24) and substitute for the rate expression for a first-order reaction under constant volume conditions, that is, - rA = kC A = kCA0 ð1- X A Þ and then simplify as shown to solve for the rate constant.

Solutions to Practice Problems

τ=

471

C A0 X A XA CA0 X A = ) k= ð - r A Þ kCA0 ð1 - X A Þ τð1 - X A Þ

Substitute all the known values into the equation for the rate constant. k=

XA 0:85 = = 1:4624 min - 1 τð1 - X A Þ ð3:875 min Þð1 - 0:85Þ

Practice Problem 6.4 Solution Since this is a gas phase reaction with unequal reactant and product moles, the volume of the reaction mixture varies with conversion. Calculate the fractional change in volume, ε, using Eq. 6.16. εA =

V XA = 1 - V XA = 0 2 - 1 = =1 V XA = 0 1

Convert the reactor temperature, which is also the feed temperature, to its absolute value. T = 460 ° þ 285 ° F = 745 ° R Calculate the initial molar concentration of A using the ideal gas law. CA0 =

P 55 psia = 0:0069 lbmol=ft3 = psia‐ft3 RT 10:73 lbmol‐0 R 7450 R

Write down the rate expression for this first-order reaction in species A and use 1 - XA Eq. 6.18, that is, C A = C A0 1þε , to obtain the following result for the rate A XA expression. - r A = kC A = kC A0

1 - XA 1 þ εA X A

Substitute the preceding rate expression into the design equation (Eq. 6.26) for a PFR and use the integration result from Eq. 6.I-3, that is,

472

6 X

0

1þεX 1 - X dX

= ð1 þ εÞ ln

1 1-X

Chemical Reaction Engineering and Kinetics

- εX, to obtain an expression for the volume of the

reactor as shown here. V τ = = C A0 v0

XA 0

v0 V= k

XA 0

kCA0

1 dX A = 1 - XA k 1 þ εA X A

XA 0

1 þ εA X A dX A ) 1 - XA

v0 1 1 þ εA X A dX A = ð1 þ εA Þ ln - εA X A 1 - XA k 1 - XA

Calculate the volume of the reactor using the ID and length of the reactor, and substitute into the preceding equation along with all the other known values and simplify as shown. V=

V=

π π 2 D L= 4 4

2

16:88 in 12 in 1 ft

ð13:8 ftÞ = 21:45 ft3 )

1 v0 - εA X A  ) ½ð1 þ εA Þln 1 - XA k

ft3 1 min ½ð1 þ 1Þln 21:45 ft3 = - ð1ÞðX A Þ ) 1 - XA 0:31 min - 1 1 - XA 0:4156 = 2ln 1 - XA 16

The preceding equation is non-linear equation in XA, and it can be solved by trial and error. Try different values of XA until the value on the right-hand side value matches with 0.4156. XA = 0.30, right-hand side =0.4139 XA = 0.302, right-hand side = 0.4175 XA = 0.301, right-hand side = 0.4157 Hence, 30.1% conversion of A is achieved in the reactor.

Solutions to Practice Problems

473

Practice Problem 6.5 Solution First, list the relevant, needed information from Example 6.9 and its solution. C A0 = 0:85 mol=L, k = 0:19 s - 1 , V = 118:79 L Note that the volume flow rate of the feed has increased from 2.5 L/s, to 3.1 L/s. This will result in a conversion lower than 90% achieved in the original reactor. The revised conversion, X1, achieved in the original reactor (which will be the first reactor in the series of two reactors) can be calculated by using the design equation for a CSTR (Eq. 6.24). V CA0 X 1 CA0 X A C A0 X 1 C A0 X 1 ) = ) = = ð - rA Þ v0, new ð - r A Þ kCA kCA0 ð1 - X 1 Þ -1 kV ð0:19 s Þð118:79 LÞ X1 = = = 7:281 ) X 1 = 0:8792 L 1 - X 1 v0, new 3:10 s τ=

Therefore, the conversion in the existing reactor has decreased as expected. Draw a schematic diagram of the two CSTRs in series as shown. Note that part of the suffix, A, used in referring to species A, has been dropped for the sake of brevity of equations.

Calculate the conversion to be achieved in Reactor 2 by using the overall conversion and the conversion achieved in the first reactor. C 2 = C 0 ð1 - X overall Þ = C 1 ð1 - X 2 Þ = C 0 ð1 - X 1 Þð1 - X 2 Þ 1 - X overall 1 - 0:95 ) 1 - X2 = = = 0:4139 ) X 2 = 0:5861 1 - X1 1 - 0:8792 Calculate the volume of the second CSTR by applying the CSTR design equation (Eq. 6.24) under the operating conditions of the second CSTR.

474

6

Chemical Reaction Engineering and Kinetics

V2 C1 X 2 C1 X 2 C1 X 2 C1 X 2 ) = ) = = ð - r2 Þ v0, new ð - r 2 Þ kC 2 kC 1 ð1 - X 2 Þ L 3:10 ð0:5861Þ v0, new X 2 s = = 23:10 L V2 = k ð1 - X 2 Þ ð0:19 s - 1 Þð1 - 0:5861Þ τ2 =

Practice Problem 6.6 Solution Determine the space time in each reactor and hence the volume of each reactor using Eq. 6.28. 1 n

1 -1 = k

1 τ= k

C0 Cn

V 1 = v k

1 ð1 - X overall Þ

v V= k

1 1 - X overall

1 n

1 n

C0 C0 ð1 - X overall Þ

1 n

-1 )

-1 )

L s -1 = 0:55 s - 1 10

1 1 - 0:98

1 5

- 1 = 21:58 L

References 1. M.E. Davis, R.J. Davis, Fundamentals of Chemical Reaction Engineering (McGraw Hill, 2003) 2. S.H. Fogler, Elements of Chemical Reaction Engineering, 6th edn. (Pearson, 2020) 3. O. Levenspiel, Chemical Reaction Engineering, 3rd edn. (Wiley, 1998) 4. J.-P. Mikkola, J.P. Wärnå, T.O. Salmi, Chemical Reaction Engineering and Reactor Technology, 2nd edn. (CRC Press, 2019) 5. G.F. Mongelli, Maximizing the intermediate product for three CSTRs in series. Research Development in Material Science (RDMS) 5(5), 501–503 (2018) 6. J.C. Pinto, M. Schmal, Chemical Reaction Engineering: Parameter Estimation, Exercises and Examples (CRC Press, 2021) 7. F. Xin, S. Li, L. Li, Reaction Engineering (Elsevier Science, 2017)

Chapter 7

Transport Phenomenon

7.1

Introduction

Transport phenomenon is a unified perspective [1, 2] of the three principal transfer processes in chemical engineering – momentum, heat, and mass transfer. Momentum transfer refers to topics in fluid mechanics, based on conservation of mass and conservation of momentum principles. The driving force in momentum transfer is pressure difference. Heat transfer is the flow of heat due to temperature difference, the driving force for heat transfer. Mass transfer covers the aspects of diffusion and transfer of mass between different entities, including the transfer of mass between different phases (interphase mass transfer). The driving force for mass transfer is the difference in concentration (or partial pressure) of the diffusing species. While the driving force causes the transfer, in each transfer process the driving force has to overcome the resistance to the transfer of momentum, heat, or mass as the case may be. Thus, we can already see the commonality between the different transfer processes in the form of presence of driving force and transfer resistance. All transfer processes can be modeled in terms of Rate of transfer /

driving force for transfer resistance to transfer

ð7:1Þ

For flow of an incompressible fluid in momentum transfer, the rate of transfer is the volume flow rate of the fluid, Q, the driving force is the pressure difference, ΔP, and the resistance to flow is due to friction losses in the conduit designated in terms of the friction factor, f. Therefore, for fluid flow, Eq.7.1 becomes Q/

ΔP f

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 N. S. Nandagopal, Chemical Engineering Principles and Applications, https://doi.org/10.1007/978-3-031-27879-2_7

ð7:2Þ

475

476

7

Transport Phenomenon

In heat transfer, the rate of transfer is the heat flow rate, q, the driving force is the temperature difference, ΔT, and the resistance to heat flow is the reciprocal of the overall heat transfer coefficient, U. Therefore, for heat transfer Eq. 7.1 becomes q/

ΔT 1 U

ð7:3Þ

In Eq. 7.3, the overall resistance to heat transfer is the sum of all the individual resistances in the given situation, that is, 1 = U

ð7:4Þ

Rindividual

In mass transfer, the rate of transfer is the molar flux of the diffusing species, NA, and the driving force is the concentration difference, ΔC, and the resistance to molar flow is the reciprocal of the overall mass transfer coefficient, K. Therefore, for mass transfer, Eq. 7.1 becomes NA /

ΔC 1 K

ð7:5Þ

In Eq. 7.5, the overall resistance to mass transfer is the sum of all the individual resistances to mass transfer, that is, 1 = K

Rindividual

The resistance to any transport process is always inversely proportional

ð7:6Þ

to the corresponding transfer coefficient:

7.2

Mass Transfer Correlations

Similar to heat transfer correlations for convection heat transfer, (introduced in Chap. 4, Sect. 4.3.3), there are mass transfer correlations [2, 3] which can be used for convective mass transfer. Consider the flow of fluid with bulk temperature, Tf, over a flat plate with surface temperature, Ts. If Ts > Tf, then heat is transferred from the plate to fluid and the relevant heat transfer correlation applicable to this situation is NuL =

1 1 hL = 0:664ðRe L Þ2 ðPrÞ3 k

Re L < 105

ð7:7Þ

7.2

Mass Transfer Correlations

477

In Eq. 7.7, NuL is the average Nusselt number over the length of the plate, L, h is the average heat transfer coefficient, k is the thermal conductivity of the fluid, ReL is the Reynolds number for flow based on the length, L, and Pr is the Prandtl number, a dimensionless number based entirely on the fluid properties, Pr = cpμ/k. Now, consider the flow of air over a pan containing water. In this situation, small amounts of water vapor will diffuse into the flowing air and this amount is governed by the moisture content of the flowing air. This is because the driving force is the difference between the saturation pressure at the water surface and the partial pressure of water vapor in air, which is related to the moisture content. The mass transfer process here has many similarities to the heat transfer process described earlier. In fact, there is an analogous mass transfer correlation given by the following equation [2, 3]. ShL =

1 1 kc,avg L = 0:664ðRe L Þ2 ðScÞ3 DAB

Re L < 5 × 105

ð7:8Þ

In Eq. 7.8, ShL is the average Sherwood number over the length of the plate, L, kc,avg is the average convective mass transfer coefficient, DAB is the diffusion coefficient of water vapor into air, ReL is the Reynolds number for flow based on the length, L, Sc is the Schmidt number, a dimensionless number based entirely on the fluid properties, Sc =

μ 1 ν μ × = = DAB DAB ρ ρDAB

ð7:9Þ

In Eq. 7.9, μ is the dynamic viscosity, ρ is the density, and ν is the kinematic viscosity of the flowing fluid. The Schmidt number used in mass transfer correlations is analogous to Prandtl number used in heat transfer correlations. The Prandtl number is the ratio of kinematic viscosity, ν, to thermal diffusivity, α, (Pr = ν/α), and the Schmidt number is the ratio of kinematic viscosity, ν, to mass diffusivity, DAB, (Sc = ν/DAB). Therefore, the similarity between Prandtl number and Schmidt number is evident. The Schmidt number can be viewed as the ratio of the thickness of the hydrodynamic boundary layer to the mass transfer boundary layer. Thus, the Schmidt number signifies the relative importance of momentum transfer over mass transfer. Example 7.1 Air at 20 °C and with relative humidity of 30% flows over a pan at a velocity of 1.70 m/s. The length of the pan is 0.85 m, and its width is 1 m. The pan contains water at 10 °C. The properties of air at 20 °C are: density = 1.2 kg/m3, kinematic viscosity = 1.55 × 10-5 m2/s. The diffusion coefficient of water vapor into air under the given conditions is 0.242 cm2/s. Determine: A. the convective mass transfer coefficient. B. the grams of water evaporated per second.

478

7

Transport Phenomenon

Solution A. Calculate the Reynolds number for air based on its free stream velocity, v1, and based on the length of the plate, L. m 1:70 ð0:85 mÞ v1 L s = 93226 < 105 Re L = = 2 ν m 1:55 × 10 - 5 s Calculate the Schmidt number using Eq. 7.9. ν = Sc = DAB

m2 s = 0:6405 1 m2 cm2 0:242 × 4 2 s 10 cm 1:55 × 10 - 5

Calculate the average value of the Sherwood number over the pan using Eq. 7.8. 1

1

ShL = 0:664ðRe L Þ2 ðScÞ3 1

1

= 0:664ð93226Þ2 ð0:6405Þ3 = 174:76 Calculate the average mass transfer coefficient, kc,avg, using the definition of the average Sherwood number given in Eq. 7.8.
ShL = k c,avg L ) DAB ð
ShL ÞðDAB Þ kc,avg = L =

ð174:76Þð2:42 × 10 - 5 0:85 m

m2 Þ s = 0:00497 m=s

B. Calculate the molar flux of water vapor diffusing into air using the average mass transfer coefficient by using the sequence of steps shown here. From steam tables, the saturation pressure of water vapor at 20 °C (the temperature of flowing air), is Psat = 2.34 kPa. The relative humidity of the flowing air is 30%, therefore, the partial pressure of water vapor (species A, the diffusing component) in the flowing air is PA1 = 0:30Psat A = 0:30 × 2:34 kPa = 0:702 kPa Calculate the absolute value of the temperature of water vapor in the flowing air.

7.2

Mass Transfer Correlations

479

T A1 = 20 ° C þ 273 ° = 293 K Calculate the molar concentration of water vapor equivalent to the partial pressure, PA1, using the molar form of the ideal gas law. PV = NRT ) C = C A1 =

PA1 = RT A1

N P ) = V RT 0:702 kPa = 2:88 × 10 - 4 kmol=m3 kJ ð293 KÞ 8:314 kmol  K

kN 2 1 kPa Units : = = m kN  m m3 kJ From steam tables, the saturation pressure of water vapor at 10 °C (the temperature of water in the pan), is PAS = 1.23 kPa. This is also the partial pressure of water vapor at the water surface. Hence, the subscript, AS. Calculate the absolute value of the temperature of water in the pan. T AS = 10 ° C þ 273 ° = 283 K Calculate the molar concentration of water vapor equivalent to the partial pressure, PAS, using the molar form of the ideal gas law. CAS =

PAS 1:23 kPa = = 5:23 × 10 - 4 kmol=m3 kJ ð283 KÞ RT AS 8:314 kmolK

Calculate the molar flux of water vapor using the mass transfer coefficient and the concentration difference driving force. N A = kc,avg ðCAS - CA1 Þ = 0:00497

m s

5:23 × 10 - 4

kmol kmol - 2:88 × 10 - 4 3 m3 m

= 1:168 × 10 - 6 kmol=m2  s Calculate the mass rate of evaporation of water using the water surface area and molecular weight of water. m_ A = ðN A ÞðAS ÞðM A Þ = 1:168 × 10 - 6 = 0:0179 g=s

kg kmol ð0:85 m × 1 mÞ 18 kmol m2  s

103 g kg

480

7

Transport Phenomenon

More Mass Transfer Correlations The mass transfer correlations for mass transfer in a pipe are 1

Sh = 0:023 Re 0:8 Sc3 , 4000 < Re < 105 , 0:6 < Sc < 3000 1

Sh = 0:015 Re 0:88 Sc3 , 104 < Re < 4 × 105 , Sc > 100

ð7:10aÞ

ð7:10bÞ

The mass transfer correlation for mass transfer between a liquid film and a gas flowing in a wetted wall column is Sh = 0:000018 Re 1:51 Sc0:5 , 103 < Re < 8:3 × 104

ð7:11Þ

In Eq. 7.11, the Sherwood number is Sh = kL,avgD/DAB and the total absorption rate can be calculated using the following equation. N A = kL,avg CA,i - CA

M

,

C A,i - C A

M

=

ðCA,i - C A0 Þ - CA,i - C A,L C - C A0 ln A,i C A,i - CA,L

ð7:12Þ

In Eq. 7.12, kL,avg is the average mass transfer coefficient over the length of the column. CA,i is the concentration of A at the interface. CA0 is the initial/entering concentration of A. C A,L is the average bulk concentration of A over the length, L. Example 7.2 Air at 70 °F and 70% relative humidity flows through a 30-ft long, 2-in pipe (ID = 2.07 in). The mass flow rate of inlet air is 0.07 lbm/sec. The pipe wall is maintained at a constant temperature of 40 °F causing some of the moisture in air to condense. The properties of air are: density = 0.075 lbm/ft3, kinematic viscosity = 1.632 × 10-4 ft2/sec, thermal conductivity = 0.0148 Btu/hr-ft-°F, heat capacity = 0.2388 Btu/lbm-°F, Pr = 0.72. The diffusivity of water vapor in air is 2.60 × 104 2 ft /sec. Determine: A. the exit temperature of air. B. the rate of condensation of water vapor. Solution A. Since the wall temperature is less than the air temperature, the air stream will cool down across the length of the pipe. Heat balance over a differential length, dL, of the pipe results in the following equation:

7.2

Mass Transfer Correlations

481

Heat transferred to the pipe wall = heat lost by air q = ha ðπD × dLÞðT a - T w Þ = m_ a cpa ð - dT a Þ ) m_ a cpa πDha

T a1

m_ a cpa πDha

T ln ðT w - T a Þ a2 -1 T a1

ln

T a2

dT a Tw - Ta

L

=

dL )

0

=L )

πDha L T w - T a1 = T w - T a2 m_ a cpa

Convert the ID of the pipe to feet. D=

2:07 in = 0:1725 ft 12 in 1 ft

To determine the heat transfer coefficient of air, calculate the Reynolds number for air. First, calculate the velocity of air using the continuity equation. va =

m_ a = ρa Acs

0:07 lbm sec 0:075

Dv Re = = ν

lbm ft3

π 4

= 39:94 ft= sec

ð0:1725 ftÞ2

ft sec ft2 sec

ð0:1725 ftÞ 39:94 1:632 × 10 - 4

= 42, 216

Calculate the Nusselt number using Eq. 4.26 (Chap. 4, Heat Transfer). Nu = Nu = ha =

0:023 Re 0:8 Pr0:3 = 0:023ð42216Þ0:8 ð0:72Þ0:30 = 104:55 ha D ) ka Btu Nu × ka 104:55 × 0:0148 hr‐ft‐ ° F = 8:970 Btu=hr‐ft2 ‐ ° F = 0:1725 ft D

Calculate the exit temperature of air by substituting all the known values into the heat balance equation.

482

7

ln

Transport Phenomenon

T w - T a1 πDha L ) = T w - T a2 m_ a cpa

40 ° F - 70 ° F ln 40 ° F - T a2

Btu ð30 ftÞ hrft2 ∘ F = 2:423 Btu lbm 3600 sec 0:07 × 0:2388 sec hr lbm∘ F

ðπÞð0:1725 ftÞ 8:970 =

Therefore, ∘

ln

- 30 F - 30 ° F = e2:423 = 11:28 ) T a2 = 43∘ F = 2:423 ) ∘ 40 ° F - T a2 40 F - T a2

B. The air leaving the pipe has to be saturated since the exit temperature of ≃43 ° F is less than the dew point of the entering air, that is, TDP1 = 60 ° F. The entrance (1) and exit (2) state points of air in the pipe are shown in the excerpt of the psychrometric chart here.

Calculate the mass flow rate of dry air. m_ da = m_ a

1 - ω1 1 þ ω1

lbm moist air s = 0:0685 lbm dry air=s

= 0:07

ð1 - 0:011Þ lbm dry air ð1 þ 0:011Þ lbm moist air

Therefore, the rate of condensation is ðm_ H 2 O Þcond = m_ da ðω1 - ω2 Þ lbm lbm H2 O lbm H2 O 0:011 - 0:006 sec lbm d a lbm d a = 0:00034 lbm H2 O=sec

= 0:0685

7.3

Analogies Between Momentum, Heat, and Mass Transfer

7.3

483

Analogies Between Momentum, Heat, and Mass Transfer

Earlier in this chapter, the similarity between Prandtl number in heat transfer and Schmidt number in mass transfer was explained. The same can be said about Nusselt number in heat transfer and Sherwood number in mass transfer. It is also important to note that the Reynolds number is an integral part of correlations in all three transport processes – momentum, heat, and mass transfer. The preceding aspects suggest the existence of analogies between momentum, heat, and mass transfer [1– 3]. Using the analogies, it is possible, for example, to predict the transport coefficient in heat transfer from the transport coefficient in momentum transfer.

7.3.1

Stanton Number for Heat Transfer

The Stanton number for heat transfer [2, 3], StH, is the ratio of the Nusselt number, Nu, to the product of Reynolds number and Prandtl number, that is, Re  Pr. The Stanton number ratio for heat transfer can be simplified by using the definitions of Nusselt number, Reynolds number, and Prandtl number that were described earlier in the chapters of Fluid Mechanics (Chap. 3) and Heat Transfer (Chap. 4), using the diameter of a pipe as the characteristic length. For flow over a parallel plate, the diameter is replaced by the length of the plate. StH =

7.3.2

Nu = Re  Pr

hD k Dvρ cP μ μ k

=

h cP ρv

ð7:13Þ

Colburn Factor for Heat Transfer

The Colburn factor for heat transfer [2, 3] is defined as 2

jH = StH Pr3 =

2 h Pr3 cP ρv

In Eq. 7.14, jH: Colburn factor for heat transfer (dimensionless) h: heat transfer coefficient (Btu/hr-ft2-°F, W/m2. °C) cP: specific heat of the fluid (Btu/lbm-°F, J/kg. °C)

ð7:14Þ

484

7

Transport Phenomenon

v: velocity of the fluid (ft/hr., m/s) ρ: density of the fluid (lbm/ft3, kg/m3) Pr: Prandtl number of the fluid Pr = cPμ/k, μ and k are dynamic viscosity and thermal conductivity, respectively, of the fluid.

7.3.3

Stanton Number for Mass Transfer

The Stanton number for mass transfer [2, 3], StM, is the ratio of the Sherwood number, Sh, to the product of Reynolds number and Schmidt number, that is, Re  Sc. The Stanton number ratio for mass transfer can be simplified by using the definitions of Sherwood number, and Schmidt number that were described earlier in this chapter and the definition of Reynolds number from Fluid Mechanics (Chap. 3), using the diameter of a pipe as the characteristic length. For flow over a parallel plate, the diameter is replaced by the length of the plate. StM =

7.3.4

k c,avg D DAB

Sh = Re  Sc

Dvρ μ

μ ρDAB

=

kc,avg v

ð7:15Þ

Colburn Factor for Mass Transfer

The Colburn factor for mass transfer [2, 3] is defined as 2

jM = StM Sc3 =

kc,avg 2 Sc3 v

ð7:16Þ

In Eq. 7.16, jM: Colburn factor for mass transfer (dimensionless) kc,avg: mass transfer coefficient (ft/sec, m/s) using the same basis as in Sherwood number v: velocity of the fluid (ft/hr., m/s) Sc: Schmidt number for mass transfer as defined earlier in this chapter. Sc =

μ ν = ρDAB DAB

7.3

Analogies Between Momentum, Heat, and Mass Transfer

7.3.5

485

Chilton–Colburn Analogy Between Momentum, Heat, and Mass Transfer

The earliest analogy between momentum, heat, and mass transfer is Reynold’s analogy. According to Reynold’s analogy the Stanton number for heat transfer as well as mass transfer is equivalent to one-half of the Fanning friction factor or one-eighth of the Darcy friction factor. Thus, Reynold’s analogy assumes the value of unity for both Prandtl number (for heat transfer) and Schmidt number (for mass transfer). T.H. Chilton and A.P. Colburn later modified Reynold’s analogy to include the effects of Prandtl number in heat transfer and Schmidt number in mass transfer. This resulted in the Chilton–Colburn analogy, which is widely used in transport phenomenon. For flow through pipes, the Chilton–Colburn analogy [2, 3] is jH = jM =

f D 4f F f = = F 8 8 2

ð7:17Þ

In Eq. 7.17, fD: Darcy friction factor fF: Fanning friction factor The following examples illustrate the applications of analogies between momentum, heat, and mass transfer. Example 7.3 The following data for flow of water in condenser tubes is available: tube ID = 41 mm, density of water = 997 kg/m3, specific heat of water = 4.180 kJ/kg. K, thermal conductivity of water = 0.6071 W/m.K, velocity of water in the tubes = 3.80 m/s, Reynolds number = 1.75 × 105, and Prandtl number = 6.12. Assuming a commercial steel pipe (pipe roughness, ε = 0.045 mm), compare the heat transfer coefficient for water obtained using: A. the heat transfer correlation for flow through pipe from Chap. 4. B. the Reynolds–Colburn analogy. Solution A. Since Re > 10, 000, calculate the Nusselt number using the Dittus–Boelter equation (Eq. 4.26, Chap. 4, Heat Transfer). The exponent n = 0.4 since water is being heated. Nu

= 0:023 Re 0:8 Pr0:4 = ð0:023Þ 1:75 × 105 = 743

0:8

ð6:12Þ0:4

486

7

Transport Phenomenon

Calculate the heat transfer coefficient for water by using the definition of Nusselt number (Table 4.1, Chap. 4, Heat Transfer). The characteristic dimension, L, in the case of a pipe is the inside diameter, Di. Nu hw

= 743 =

hw Di ) k

W 743 × k ð743Þ 0:6071 m  K = = = 11002 W=m2  K Di 0:041 m

B. Calculate the Colburn jH factor in terms of h, the heat transfer coefficient, using Eq. 7.14, and substituting all the known values.

jH

=

2 h Pr3 = ρcP v

h 997

kg m3

4:180 × 103

2

J kg  K

3:80

m s

ð6:12Þ3

= 2:11 × 10 - 7 h Calculate the relative roughness using the pipe roughness and ID. r=

ε 0:045 mm = = 0:0011 D 41 mm

Using the Reynolds number (1.75 × 105) and relative roughness (r = 0.0011) as parameters, determine the Fanning friction factor from Fig. 3.6 as shown in the figure.

From the figure, fF = 0.006. Calculate the heat transfer coefficient using the Chilton–Colburn analogy (Eq. 7.17). jH =

fF 0:006 ) 2:11 × 10 - 7 h = ) h = 14218 W=m2  s 2 2

7.3

Analogies Between Momentum, Heat, and Mass Transfer

487

The value of the heat transfer coefficient obtained from Chilton–Colburn analogy is higher than the value obtained from the heat transfer correlation. This is because the accuracy of results obtained from Chilton-Colburn analogy is highly dependent on the accuracy and value of the friction factor obtained from the friction factor chart. The friction factor is dependent on the pipe roughness, which is used in the analogy but not usually factored in heat transfer correlations. Example 7.4 Water flows in a 7.98-in ID steel tube (pipe roughness, ε = 0.0018 in.) at a velocity 11 ft./sec. The temperature of water increases from 50 °F to 52 °F over a tube length of 12 ft. by virtue of the walls of the tubes being maintained at 100 °F. The relevant properties of water are density = 62.2 lbm/ft3, kinematic viscosity = 9.30 × 10-6 ft2/ sec, specific heat = 0.998 Btu/lbm-°F, thermal conductivity = 0.3520 Btu/hr-ft-°F, Prandtl number = 5.96. Calculate the heat transfer coefficient required for heating the water by using energy balance and compare this calculated value with that predicted by the Chilton–Colburn analogy. Solution Convert the inside diameter of the tube to feet. D = 7:98 in ×

1 ft = 0:665 ft 12 in

Determine the mass flow water by using the continuity equation (Eq. 3.5) lbm π ð0:665 ftÞ2 3 4 ft = 237:64 lbm= sec

m_ water = ρwater Avwater = 62:2

11

ft sec

Energy (heat) balance across a differential length, dL, of the tube results in the following equation: Heat lost by the tube wall = heat gained by water ðhÞðdAÞðT wall- T water Þ = m_ water cp,water dT water The differential surface area corresponding to a differential length, dL, of the duct is the perimeter of the tube multiplied by the differential length. dA = P × dL = πDdL = π ð0:665 ftÞdL = 2:09 ft × dL Substitute for dA into the heat balance equation, separate the variables, and then integrate with appropriate limits.

488

7

Transport Phenomenon

ðhÞð2:09 ft × dLÞðT wall- T water Þ = m_ water cp,water dT water L

m_ water cp,water dL = 2:09 ft × h

T water,out

dT water T wall - T water T water,in

0

L=

m_ water cp,water T wall - T water,in ln 2:09 ft × h T wall - T water,out

Solve the preceding equation for h and calculate h by substituting all the known values. m_ water cp,water T wall - T water,in ln 2:09 ft × L T wall - T water,out lbm Btu 3600 sec 237:64 × 0:998 ∘ × 100∘ F - 50∘ F sec hr lbm F = ln 2:09 ft × 12 ft 100∘ F - 52∘ F

h=

= 1, 390 Btu=hrft2 ∘ F To calculate the heat transfer coefficient by using the Chilton–Colburn analogy, determine the Reynolds number by using Eq. 3.7. Re =

ft ð0:665 ftÞ 11 sec Dvwater = = 7:9 × 105 2 νwater ft 9:30 × 10 - 6 sec

Calculate the relative roughness using the pipe roughness and ID. r=

ε 0:0018 in = = 0:0002 D 7:98 in

Using the Reynolds number (7.9 × 105) and relative roughness (r = 0.0002) as parameters, determine the friction factor from Fig. 3.6 as shown in the figure.

From the figure, f = 0.0038. Calculate the Colburn jH factor in terms of h, the heat transfer coefficient, using Eq. 7.14 and substituting all the known values. Note that

7.3

Analogies Between Momentum, Heat, and Mass Transfer

489

since the units of the heat transfer coefficient is usually Btu/hr-ft2-°F, it is necessary to have the velocity in ft./hr.

jH

=

2 h Pr3 = cP ρv

Btu 0:998 lbm ° ‐F

h lbm 62:2 3 ft

2

ft 3600 s 11 × s hr

ð5:96Þ3

= 1:33 × 10 - 6 h Calculate the heat transfer coefficient using the Chilton–Colburn analogy (Eq. 7.17). jH =

∘ fF 0:0038 ) 1:33 × 10 - 6 h = ) h = 1429 Btu=hrft2  F 2 2

Again, the value of the heat transfer coefficient obtained from the Chilton– Colburn analogy is higher than the value obtained from the heat transfer correlation. This is because the accuracy of results obtained from Chilton-Colburn analogy is highly de-pendent on the accuracy and value of the friction factor obtained from the friction factor chart. The friction factor is dependent on the pipe roughness, which is used in the analogy but not usually factored in heat transfer correlations. Example 7.5 Use the data for the flow of air over a flat plate considered in Example 7.1. For laminar flow over a flat plate, the following relationship is available for the skin friction factor. Cf =

1:33 , Re 0:50 L

Re L < 5 × 105

ð7:18Þ

A. Develop an expression for the Colburn jM factor for flow over a flat plate in terms of the skin friction factor for flow over a flat plate. B. Use the expression developed in part A to predict the mass transfer coefficient for the conditions given in Example 7.1. Solution A. Start with the mass transfer correlation for flow over a flat plate (Eq. 7.8). ShL =

1 1 kc,avg L = 0:664ðRe L Þ2 ðScÞ3 DAB

Divide both sides of the preceding equation by ReL  Sc)

490

7

ShL Re L  Sc

1

= StM =

Transport Phenomenon

1

0:664ðRe L Þ2 ðScÞ3 ) Re L  Sc - 12

2

= 0:664 Re L

StM Sc3

From Eq. 7.16, the left-hand side of the preceding equation is the Colburn jM factor for mass transfer. Therefore, 2

- 12

StM Sc3 = jM = 0:664 Re L

Combine the preceding equation (Chilton–Colburn analogy for flow over a flat plate) with the equation provided for determining the skin friction factor, that is, to obtain the following expression for the Colburn jM factor C f = 1:33= Re 0:5 L for flow over a flat plate in terms of the skin friction factor.

jM = StM Sc3 = 0:664 Re L- 0:5 = 2

0:664 1:33

1:33 = 0:4992Cf ) jM = 0:4992C f Re 0:5 L

B. The relevant data and results from Example 7.1 are: v = 1:70 m=s, ρ = 1:2 kg=m3 , ν = 1:55 × 10 - 5 m2 =s, L = 0:85 m DAB = 0:242 cm2 =s, ReL = 93226, Sc = 0:6405 Calculate the skin friction for the flow of air over the plate using Eq. 7.18 and the calculated value of the Reynolds number from Example 7.1. Cf =

1:33 1:33 = = 0:0044 Re 0:50 ð93226Þ0:50 L

Substitute this result into the expression derived for Colburn jM factor for flow over a flat plate and hence calculate the Colburn jM factor. jM = 0:4992C f = ð0:4992Þð0:0044Þ = 0:0022 Substitute the preceding result into Eq. 7.16, solve for the mass transfer coefficient, and hence calculate the mass transfer coefficient for this situation. jM =

kc,avg 2 Sc3 ) v

m jM × v ð0:0022Þ 1:70 s k c,avg = = = 0:0050 m=s 2 2 Sc3 ð0:6405Þ3

7.3

Analogies Between Momentum, Heat, and Mass Transfer

491

The mass transfer coefficient predicted by the Chilton–Colburn analogy compares very well with the calculated value, 0.00497 m/s, from Example 7.1. Hence, the skin friction factor for flow over a flat plate can be effectively used to predict the mass transfer coefficient in mass transfer during flow over flat plate. Example 7.6 A. Predict the heat transfer coefficient and the mass transfer coefficient for the situation described in Example 7.2 using the Chilton–Colburn analogy for pipe flow (Eq. 7.17), assuming the use of a commercial steel pipe. B. Calculate the mass transfer coefficient using the mass transfer correlation for flow through a pipe. C. Compare the predicted values for the coefficients with the values calculated from the transfer correlations. Solution The data and results from Example 7.2 that are relevant to this problem are: Properties of air: density = 0.075 lbm/ft3, kinematic viscosity = 1.632 × 10-4 ft2/sec, thermal conductivity = 0.0148 Btu/hr-ft-°F, heat capacity = 0.2388 Btu/lbm-°F, Pr = 0.72, diffusivity of water vapor in air = 2.60 × 10-4 ft2/sec, velocity of air = 39.94 ft./sec, Re = 42,216, Sc = .6277, pipe ID = 2.07 in = 0.1725 ft. Calculate the relative roughness of the pipe surface using the ID of the pipe and the standard roughness of 0.0018 in for a commercial steel pipe. r=

ε 0:0018 in = = 0:0009 D 2:07 in

Using the Reynolds number (42,216  4.2 × 104) and relative roughness (r = 0.0009) as parameters, determine the friction factor from Fig. 3.6 as shown in the figure.

From the figure, f = 0.0064. A. Heat Transfer Calculate the Colburn jH factor using the Chilton–Colburn analogy (Eq. 7.17) and the value of the friction factor just obtained.

492

7

jH =

Transport Phenomenon

fF 0:0064 = = 0:0032 2 2

Solve Eq. 7.14, for h, the heat transfer coefficient, and calculate h by substituting all the known values. Note that since the units of the heat transfer coefficient is usually Btu/hr-ft2-°F, it is necessary to have the velocity in ft./hr. jH =

h=

2 h Pr3 ) cP ρv

jH cP ρv = 2 Pr3

ð0:0032Þ 0:2388

Btu lbm ° ‐F

0:075

lbm ft 3600 s × 39:94 × 3 s hr ft 2

ð0:72Þ3

= 10:26 Btu=hr‐ft2 ‐ ° F Mass Transfer Calculate the Schmidt number for the diffusion/mass transfer of water vapor in the pipe using Eq. 7.9. ft2 1:632 × 10 - 4 ν s = 0:6277 Sc = = 2 DAB - 4 ft 2:60 × 10 s Calculate the Colburn jM factor using the Chilton–Colburn analogy (Eq. 7.17) jM =

fF 0:0064 = = 0:0032 2 2

Using the preceding value of Colburn jM factor, calculate the mass transfer coefficient for the given situation using Eq. 7.16. jM =

k c,avg 2 Sc3 ) v ð0:0032Þ 39:94

j ×v k c,avg = M 2 = Sc3

2

ð0:6277Þ3

ft s

= 0:1743 ft=s

B. Calculate the Sherwood number using the mass transfer correlation for mass transfer in a pipe (Eq. 7.10a). Use the value of Schmidt number already calculated in part A. 1

1

Sh = 0:023 Re 0:8 Sc3 = 0:023ð42216Þ0:8 ð0:6277Þ3 = 98:79

References

493

Calculate the average mass transfer coefficient using the definition of the Sherwood number. Sh =

k c,avg D ) DAB

kc,avg =

Sh × DAB = D

ð98:79Þ 2:60 × 10 - 4 0:1725 ft

ft2 sec

= 0:1489 ft= sec

C. For heat transfer. h = 8.97 Btu/hr ‐ ft2 ‐ ° F from heat transfer correlation. h = 10.26 Btu/hr ‐ ft2 ‐ ° F from Chilton–Colburn analogy. Percent difference = 14.4. For mass transfer kc, avg = 0.1489 Btu/hr ‐ ft2 ‐ ° F from mass transfer correlation. kc, avg = 0.1743 Btu/hr ‐ ft2 ‐ ° F from Chilton–Colburn analogy. Percent difference = 17.1.

References 1. B.R. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomenon, 2nd edn. (Wiley, 2008) 2. C.J. Geankoplis, A. Hersel Allen, D.H. Lepek, Transport Processes and Separation Process Principles, 5th edn. (Wiley/Pearson Education, 2018) 3. J. Welty, G.L. Rorrer, D.G. Foster, Fundamentals of Momentum, Heat, and Mass Transfer, 7th edn. (Wiley, 2017)

Index

A Absorptivity, 307–308 Activation energies, 446–448, 467 Activity coefficients, 116, 117, 125, 126, 130–132, 138, 151, 166 Adsorbate, 399 Adsorbents, 399–402, 424 Adsorption, vi, 399–405 Air-fuel ratio, 48 Arrhenius equation, 445–448 Azeotropes, 129–132

B Batch reactors, 9, 445, 450–454, 467, 469 Bernoulli’s equation, 214, 226 Bubble point, 122–128, 153 Bypasses, 21–23, 56, 70, 71, 175, 202, 253, 260, 308

C Chemical equilibrium, v, 81, 136–148 Chemical thermodynamics, v, 80, 81 Chilton–Colburn analogy, 485–493 Clausius–Clapeyron equation, 134–136 Colburn j factors, 486, 488–492 Combustion products, 39, 45, 48–52 Combustion reactions, 45–54 Combustion stoichiometry, 47–49 Compressibility factor, 85–89, 156, 157 Compressible flows, 186, 240–251 Conduction, 270–280, 282, 286–289, 292, 293, 324

Constant rate period, 370, 373 Continuity equation, 185–187, 189, 193, 196, 199, 201, 202, 212, 240, 257, 284, 481, 487 Continuous stirred tank reactor (CSTR), 451, 454–459, 462–468, 470, 472, 473 Convection, 270, 271, 274, 275, 280–290, 292, 311, 312, 327, 369, 476 Cooling towers, 361, 365–368 Crystallization, vi, 323, 405–410 CSTRs in series, 464, 466, 468, 473

D Design equations, 287, 294–298, 300, 301, 316, 317, 409, 413, 416–418, 450–467, 469–473 Dew point, 51, 58, 80, 122–128, 152, 167, 168, 362, 363, 482 Dew point of combustion products, 51 Differential manometers, 180, 182–185, 221–223 Diffusion coefficient, 324, 477 Diffusivity, 282, 324, 477, 480, 491 Dimensionless number, 188, 477 Distillation, vi, 3, 112, 118, 129, 175, 185, 229, 313, 323, 345–361, 421 Double pipe heat exchangers, 199, 200, 282, 283, 290, 295, 297 Double/triple-effect evaporators, 410, 413 Drying, vi, 56, 323, 361, 369–376, 422, 431, 432, 434

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 N. S. Nandagopal, Chemical Engineering Principles and Applications, https://doi.org/10.1007/978-3-031-27879-2

495

496 E Effectiveness-NTU method, 301 Emissivity, 307, 309, 311, 314 Equations of state, 81–92 Equilibrium constant, 137–143, 145, 147, 148, 154, 171–173 Equilibrium line/curve, 333, 334, 352–354, 356–358, 383, 385, 403, 405, 435 Equivalent length method, 205–206 Evaporators, 56, 70–72, 295, 297–299, 410–420 Excess air, 47–51 Extract, 56, 70, 377–381, 383, 384, 386–390, 410, 423, 435, 436

F Falling rate period, 370, 371 Fanning friction factors, 190–198, 201, 203, 248, 249, 252, 253, 259, 485, 486 Feed condition factor, 351–352 Feed line, 350 Flue gas, 49–51, 91 Flue gas analysis, 2 Fluid mechanics, v, 175, 181, 283, 325, 331, 338, 475, 483, 484 Fractional change in volume, 449, 450, 471 Fractional conversion, 7, 444, 445, 449, 450, 467 Frequency factors, 446, 447, 467 Friction head loss, 190–199, 202–206, 210, 212–214, 216, 217, 259 Fundamental energy balance equations, 30–37, 41 Fundamental mass balance equation, 1, 2

G Gas absorption, vi, 175, 229, 235, 236, 323, 331–344, 384, 401 Generalized pressure drop correlations (GPDC), 232–236, 331 General procedure for mass balances for reactive systems, 40–45 Gibbs free energy changes, 133–136, 145–148

H Heat balance, 291, 295–299, 301, 315, 319, 406, 480, 481, 487 Heat capacity, 282, 301–304, 312, 317, 318, 480, 491

Index Heat exchanger effectiveness, 300–302, 304, 305, 318 Heat transfer, 28, 30–32, 34, 38, 41, 44, 45, 58, 77, 108, 242, 269–292, 300–302, 305–313, 315, 317, 319, 325, 327, 337, 361, 369, 406, 414, 418, 475–477, 481, 483–486, 491, 493 Heat transfer coefficients, 280, 281, 283–290, 292–295, 297, 299, 300, 302, 311–313, 315, 317, 327, 406, 411, 414, 476, 477, 481, 483, 485–489, 491, 492 Heat transfer correlations, 282, 283, 312, 476, 477, 485, 487, 489, 493 Heat transfer modeling, 270 Heat transfer resistances, 281–282, 286, 288, 327 Heat transfer surface area, 294, 297, 302, 317, 411, 413, 416 Height of transfer unit (HTU), 331, 337–344 Humidification, vi Hydraulic diameter, 199, 200, 202, 283–285

I Incipient fluidization, 237–240, 254, 264 Individual mass transfer coefficients, 326, 327, 420 Initial–change–equilibrium (ICE) method, 141 Isentropic flows, 242–248

L Leaching, vi, 323, 389–398, 423 Le-Chatelier’s principle, 144–145 Liquid-liquid extraction, vi Log mean temperature difference (LMTD), 291, 295, 299–300, 337

M Mach number, 240–242, 244–250, 254 Mass balances for combustion reactions, 45 Mass balances for non-reactive systems, 2, 23 Mass balances for reactive systems, 2, 4 Mass balances for steady-state non-reactive systems, 2 Mass balances for steady-state reactive systems, 2 Mass balances for systems with bypass, 21–23 Mass balances for systems with recycle, 14–21 Mass balances for unsteady-state systems, 23 Mass transfer, v, vi, 81, 175, 229, 323, 325–331, 333, 337, 352, 361, 369,

Index 389–391, 424, 425, 475–477, 480, 483–493 Mass transfer coefficients, 325–331, 337, 338, 342, 344, 370, 390, 391, 477–480, 484, 489–493 Mass transfer correlations, 476–482, 489, 491–493 McCabe–Thiele method, 350, 361 Mechanical energy equation, 209–221 Minimum fluidization velocity, 237, 254, 264 Minimum liquid-gas ratio, 331, 333 Minimum reflux, 350, 352, 354, 356–358, 421, 430 Minimum theoretical stages, 361 Minor losses, 203–209 Mixed flow reactor (MFR), 450, 451, 454–459, 462–464 Molar heat capacities, 42 Molar ratio of air to oxygen, 24 Molar ratio of nitrogen to oxygen, 48 Molecular diffusion, 324–325 Mollier diagrams, 103–106, 160–162 Momentum transfer, 175, 325, 475, 477, 483 Momentum-heat transfer analogy, 175 Momentum-mass transfer analogy, 175 MTD correction factors, 299, 300

N Net radiation heat transfer, 306, 308–311 Non-reactive systems, 24 Number of transfer units (NTU), 301–305, 318, 331, 337–344, 384–386 Nusselt number, 285, 481, 483, 485

O Operating lines, 333, 349, 350, 356, 358, 360, 403, 405, 430, 431 Order of reaction, 442, 443 Orifice meters, 175, 221, 223–225, 228, 253 Orsat analysis, 49, 50 Overall heat transfer coefficients and resistance, 286–288 Overall mass transfer coefficient, 326, 328, 476 Overflow solution, 394, 398, 424

P Parallel pipes, 202 P–h diagrams, 94, 107, 108, 111 Pipe networks, 202–203 Pitot tubes, 221–223

497 Plug flow reactor (PFR), 450, 451, 459–464, 467, 471 Prandtl numbers, 282, 285, 314, 477, 483–485, 487 Pressure drop across packed beds, 230, 237 Pressure drop for fluid flow in pipes, 190 Psychrometric chart, 361–365, 367, 374–376, 434, 482 Psychrometrics, 361–365 Pump characteristics, 219

Q q line, 350–352, 354, 358

R Radiation, 270, 271, 305–312, 314, 320, 369 Radiation view factor, 308 Raffinate, 377–381, 383–387, 389, 423, 435, 436 Raoult’s Law, 112–115, 118, 164 Rate constants, 442–448, 453, 456, 458, 461, 462, 466–468, 470 Rate expressions, 442, 443, 452, 453, 456, 458, 459, 461, 463, 464, 466, 469–471 Reaction rates, 441, 442, 444–446, 452, 453, 461, 466, 467 Reactive systems, 4–14, 40, 399 Rectification section, 349, 358, 430 Recycle, 14–21, 56, 63, 65–67, 175 Reflectivity, 307–308 Reflux ratios, 349, 352–354, 357, 358, 421 Reynolds analogy, 485 Reynolds number, 188–189, 191, 193–196, 198, 199, 201, 206–208, 212, 230, 231, 252, 253, 262, 284, 370, 477, 478, 481, 483–486, 488, 490, 491

S Schmidt number, 477, 478, 483–485, 492 Shape factor algebra, 308 Shell and tube heat exchangers, 187, 299, 302, 313, 410 Sherwood number, 478, 480, 483, 484, 492, 493 Single-effect evaporator, 410–413, 419, 420 Solute, 115, 331–333, 335, 340, 341, 343, 376, 377, 380–383, 385, 386, 388–402, 405, 423, 424, 428, 436, 437 Solution, 3, 6, 9, 12, 15–19, 22, 24, 29, 32–36, 38, 40, 42, 46, 48, 50–53, 56–81, 83, 86,

498 91, 98, 101, 103, 105, 108, 111–118, 121, 124, 125, 127–131, 136, 140, 141, 143, 147, 148, 150, 151, 155–174, 178, 184, 187, 189, 191–193, 196, 199, 200, 203, 206, 212, 216, 218, 220, 222, 225, 227, 228, 230, 233, 238, 243, 246, 249, 254–267, 273, 274, 277, 279, 281–283, 288, 295, 298, 299, 303, 310, 311, 314–320, 324, 328, 329, 331, 334, 338, 340–342, 346, 355, 358, 361, 365, 367, 373, 374, 376–378, 380–382, 384–388, 391, 393, 394, 396–398, 400, 402, 403, 405–407, 409–417, 421, 423–439, 446, 447, 453, 456–458, 461, 463, 466, 468–474, 478, 480, 485, 487, 489, 491 Solvent, 115, 232, 254, 323, 326, 328, 331–333, 338, 341, 343, 376–380, 382, 384, 385, 387–390, 392, 393, 397–399, 421, 423, 424, 427, 435, 437 Space time, 455, 465, 470, 473 Space velocity, 455 Specific heat, 36, 241, 243, 282, 283, 291, 295, 299, 302, 313, 354, 406, 411, 457, 483, 485, 487 Stack gases, 49, 52, 80, 91, 92 Standard heat of combustion, 39, 46, 54 Standard heat of formation, 37, 41, 42 Standard heat of reaction, 37–39, 41, 53, 58, 74, 75, 77, 147, 148 Stanton number, 483–485

Index Steady-state energy balance, 31–37, 311 Steady-state mass balance, 12, 59 Steam tables, 28, 29, 32, 53, 80, 93–94, 97–104, 106, 133, 154, 171, 298, 315, 362, 367, 408, 412, 414–417, 419, 478, 479 Stefan–Boltzmann’s law, 306–307 Stripping, vi, 328, 331–344, 354 Stripping sections, 347, 349, 350, 358, 431

T Theoretical air, 48, 49, 51 Thermochemical data, 37–39, 42 Total reflux, 353, 354, 356 Transmissivity, 307–308 Transport phenomenon, vi, 475, 485

U Underflow solutions, 393, 394, 396, 398, 424, 437 Unsteady-state mass balances, 24, 25, 72, 73 Useful integrals, 450

V Vapor liquid equilibrium (VLE), v, 112–132, 356 Velocity head method, 203–206 Venturi meters, 225–228, 253